email

• 電子メール

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electronic mail

WordNet

1. a service providing free email delivery in exchange for exposure to advertising

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/10/07 11:01:36」(JST)

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「@」（単価記号、アットマーク）はすべてのSMTP メールアドレスで使われる

電子メール、eメール（英: electronic mail,e-mail）は、コンピュータネットワークを使用して、郵便のように情報等を交換する手段である。

概要

インターネットの初期からある通信手段であり、UUCPやSMTPなどのプロトコルを介して、メールを相手サーバに届けられる。電気的な信号で送受信を行うのでかかる時間は数分程度である。

一方で、インターネットの普及以前にコンピュータ通信手段として広く行われていた、いわゆるパソコン通信でも、加入者同士で文書のやり取りを行うシステムが「電子メール」として提供されていた。ただし、パソコン通信では、一般的に、通信が1つのパソコン通信システム内にとどまっていたので、他のシステムとの間での電子メールの交換機能などの相互通信機能はほとんどなかった。また、各パソコン通信システムごとに独自のシステムが構築されていた事が多かったので、ユーザーインターフェイス等についても互換がなかった。しかしその後、インターネットの普及に伴って、大手パソコン通信システムとインターネット間で相互に通信が可能にもなった。メール友達（メル友）も、流行になった時期があった。

インターネットが普及し始めた頃（あるいは現在も）は電子掲示板(BBS)の書き込みやブログのコメントさえも含めて、「メール」と呼称していた初心者が多かった。

また、携帯電話やPHS間でごく短い文章を送受信する、ショートメッセージサービス（SMS。iモードなどのサービス開始前より行われている）も、広義の電子メールに含まれる。

クライアント環境にウェブブラウザ以外のソフトウェアを必要としないウェブメールも広義の電子メールであり、これを用いたフリーメールサービスも普及している。

なお、以下ではRFCに準拠した、UUCP、SMTPのプロトコルを使用した電子メールについてのみ記述する。それ以外の電子メールについては上記の各関連項目を参照のこと。

電子メールを支える技術

一般

個々の電子メールのアドレスは、「Yamada.Tarou@example.wikipedia.org」などのような形で表現される。実際に電子メールを使うためには独自ドメイン名（この例では「example.wikipedia.org」）を得て、ドメイン名を管理するDNSサーバーやメールサーバーに登録する必要があることがほとんどである。

一般的には、加入インターネットプロバイダーや勤務先・通学先の企業・学校などのアドレス（アカウント）になっていることが多い。

容量については理論的には制限はないが、送受信可能な最大容量は、プロバイダーの提供する容量で制約を受ける。一般的には、ダイヤルアップ接続時代の名残の数メガバイト(MB)から、近年のブロードバンド対応として大容量を謳ったものでは100MB～数GB(ギガバイト)程度に設定されることが多い。主要プロバイダの一例としてはOCNが10MBまで、So-netが20MBまで、Biglobeが100MBまでである。これ以上の大容量のデータのやり取りにはFTPやP2P、HTTP等によるオンラインストレージ、ファイル転送サービス、アップローダーなどを使用する。

無料アドレス（フリーメールサービス）の場合は、プロバイダーなどのアカウントで利用する一般的な電子メールクライアントソフトではなく、ウェブブラウザを使ってウェブページ上で、送受信を行うウェブメールがほとんどである。

プロトコル

現在、インターネットでは、メールサーバ間での通信およびクライアントからの送信には、一般にSMTPが使われる。古くは、また現在でも希に、UUCPが使われる。メールは、数々のサーバーをリレーのように経由して目的のメールサーバに伝えられる。なお、電子メールには、送信者の使用メールソフトや経由サーバーなどのヘッダーと呼ばれる情報が付属されている。

メールサーバからメールを読み出す場合には、POP、IMAPなどのプロトコルが用いられる。メールの書式については、RFC 5322で規定がある。また、英字以外の文字・言語やテキスト以外の情報をメールで送るなどのためにMIMEが規定されている。

文字コード

元来のメールの文字コードはUS-ASCIIのみであったが、上記MIMEの規定により様々な文字コードが使えるようになった。

かつての日本のJUNETではJIS規格に基づく規則を決めて日本語を扱えるようにした。この規則をMIMEの枠組みで再定義したものがISO-2022-JPである。現在の日本語メールでは、このISO-2022-JPが広く用いられている。

RFC 2277では、出来るだけ広く知られた文字コードを選ぶように注意を促している。これはUTF-8が普及するまでの暫定的なものであるが、その期間は50年であるかもしれないので事実上は永遠と考えてよいとも書かれている。

メール形式

元来は、メールは文章程度のプレーンテキスト形式の物のみであったが、上記MIMEの規定および普及に伴って、メール本文をHTMLにより記述したHTML形式のメールも、RFCに規定されて一般にも使われるようになった。

HTML形式のメールは、メール本文がHTMLで記述できるのでメールにWebページと同様の表現力を持たせられる利点がある。携帯電話・PHSでも、cHTML形式のメールが一般向け仕様のサービスとして提供されているものもある。

その一方で、特に、Microsoft Windows と、その標準メールクライアントである Outlook Express（初期設定ではメールの作成時にHTML形式が選ばれる）の普及に伴って、HTML形式のメールが送受信されることも多くなった。しかしながらメールクライアントにおいては、メール中のHTML情報を展開し表示するためのレンダリングエンジン（Internet Explorerをはじめとするウェブブラウザ）にしばしばセキュリティホールが発見されているので、メールを見る（プレビューする）だけで、コンピュータウイルスが侵入する被害を受けたり、迷惑メール・架空請求メール等で画像タグを埋め込んだメールを送りつけて表示させて、メールを表示させた情報を収集（ウェブビーコンと言う）して悪用するなど、セキュリティ上の問題がある（ただし「HTMLメールを表示する事」は「ブラウザでWebページを表示する事」と、技術的には根本的な違いはない）。対策としては、ウイルス対策・迷惑メール対策のソフトを導入するか、HTML形式のメールをフィルタリング機能で受信を拒否する・ゴミ箱フォルダへ振り分けるなどがある。また、HTMLメールの表示に対応していないメールクライアントもあって、断り無くHTML形式のメールを送信しても正しく受信されないおそれがある。

なお、あるファイルデータをメールに添付して送る場合、添付ファイルとしてMIMEなどによってテキスト化（エンコード）をしてメール本文に埋め込んで送信して、受信側で元のデータファイルに復元（デコード）する方法が取られる。添付ファイルには、コンピュータウイルスも仕込み可能なので、受信時に添付ファイルを自動的に開く設定になっていると、やはりコンピュータウイルスが侵入する被害を受けるなどの危険もある。

ヘッダー情報

一通一通それぞれのメールは、本文とは別に、ヘッダーフィールドと呼ばれる各種の特殊な情報が記載された領域を持つ。ほとんどのメールクライアントでは、何らかの方法（メールクライアント毎に異なる）によって、このヘッダーフィールドの情報を参照可能である。この情報は、脅迫メールやスパムなどのメールが届く場合などに、送信元の特定などに威力を発揮する。ただし、偽装も可能で必ずしもすべてのヘッダフィールドを付加する必要はないので、完全には判断できない。

代表的なヘッダフィールド

ヘッダーフィールドは フィールド名:フィールド値 という形で記載される。

Bcc

Cc

Ccはカーボンコピーの受取人、Bccはブラインドカーボンコピーの受取人のメールアドレス。単数や複数の名前やアドレスも含められる。#CcとBccを参照。

Date

送信者が送信を行った日時

From

著者のメールアドレス。単数または複数の名前やアドレスも含められる。

このヘッダーの記載は送信者がメールクライアントの設定によって自由に変更できる。このようなメールの仕様から、いわゆる「なりすまし」などの悪用を完全に防ぐことは困難とされる。

In-Reply-To

返信元メールなどのMessage-IDの値の一覧

Message-ID

メール一通一通に付加された固有の番号

MIME-Version

MIMEのバージョン

Received

このメールが届くまでに経由したメール転送エージェント（IPアドレス）および経由した日時

Reply-To

送信者が返信先として希望するメールアドレス

Return-Path

SMTP通信で送信元として伝えられるメールアドレス

Sender

送信者のメールアドレス。名前も含められる。著者と送信者が同一、すなわちFromが単一のアドレスでSenderと同じ場合は使うべきではない。逆に、異なる場合は必須である。

Subject

話題を表す短い文。日本語では件名、サブジェクトなどと呼ばれる。返信の場合はRe:、転送の場合はFw:が先頭に自動的に付加される場合が多い（#ReとFwを参照）。

To

受取人のメールアドレス。単数または複数の名前やアドレスも含められる。

X-FROM-DOMAIN

送信者のドメイン

X-IP

送信者のグローバルIPアドレス

X-Mailer

メールクライアントの種別

X-Priority

送信者が指定した重要度

保存形式

• eml形式：1メール1ファイル
• mbox：一つのファイルに複数のメール（1フォルダ1メールなど）
• Maildir

機能

CcとBcc

メールを送信する際の機能として、Cc（カーボンコピー）とBcc（ブラインドカーボンコピー）の2種類ある。メールの本来の送信先は一般的にTo:に指定して送信するが、本来の送信先以外にも一応複製を送っておきたい相手などがいるという場合にこの機能を使用する。

メールを初めて利用する人はもちろん、それなりに使い慣れている人にしても、この機能の本来の使用方法を理解していない事も多い。この機能を使うに当たっては、よく理解して使えばとても便利であるが、私用・公用に限らず、Cc機能とBcc機能の違い・それぞれに指定されて送信された相手に見える自分以外の送信先をよく理解して使わないと、例としてメールアドレスの個人情報漏洩など、色々な意味で問題を起こす事となる。

また、2010年現在でも、Bccとして指定したメールアドレスを他の受信者に見せてしまったり、ヘッダー内の別領域に書いてしまったりする困った障害を起こすソフトウェアが存在するので、Bcc機能を理解していてもあえて使わない利用者も居る。

Cc

Toで指定した本来の送信先以外にも、一応複製を送っておきたい相手などがいる場合に使用する機能である。

Toで宛先を指定するのと同様に、Ccに複製を送りたい相手を指定して使用する。Toに指定された本来の相手には、ToとCcに指定された宛先が全て見える。また、Ccに指定された相手にも、ToとCcに指定された宛先が全て見える。

要するに、送信者 (From)、Toの相手、Ccの相手、の各3人相互で、各アドレスが各3人全員に知られることになる。

Bcc

Toで指定した本来の送信先以外にも一応複製を送っておきたい相手がいるが、ToとCcに指定した相手には、複製を送信した相手のメールアドレス及び複製を送信した事実を知られたくない場合などに使用する機能である。

Toで宛先を指定するのと同様に、Bccに複製を送りたい相手を指定して使用する。メールの送信時に、メールサーバー (MTA) においてBccヘッダーを削除して転送するので、To、Ccに指定された相手には、このBccに指定された宛先は全く見えない。しかし、Bccに指定された相手には、ToとCcに指定された宛先が全て見える。また、Bccの宛先アドレスが複数ある場合には、Bcc指定された各宛先相互間で、自分以外の他の宛先は分からない。

複数のメールクライアントから単一のメールアカウント・サーバーに接続する場合には、Bccを活用した技がある。BccにFrom（自分自身）と同じアドレスを指定する（メールクライアント (MUA) による常時設定も可能）事によって、自分が送信したメールがそのままの内容で自分のメールクライアントの受信箱にも配信される。POP3等のメールサーバーでサーバーかメールクライアントへ受信したメールをサーバーから除去しない（数日後に削除する）設定をメールクライアントにすることによって、1つのメールクライアントから送信したメールが他のメールクライアント全てに複製として配信される。これによって、通常は送信したメールクライアントの送信済み箱を見ないと分からない所が、複数のメールクライアントで送信メールが確認できる。

ネチケットの一つとして推奨されてきたメールの送信方法であるが、一斉メールはどのような場合でもBccを使用するべきかといえばそうでもない。例えば特定の一斉送信されたメールについて、全ての受信者がメールアドレスを交換し合っている場合にはBccを使う必要性はなく、どちらかというと宛先と目的がはっきりと明示されているToとCcを使いわけるのが普通である。時と場合によりTo、Cc、Bccを適切に使い分けるためには高度なネチケット知識が必要である。

なお、時々「ブラックカーボンコピー」と言われることがあるが、これは間違いである。

ReとFw

Re（返信）

多くの電子メールクライアントでは、返信されたメールの件名の先頭に自動的にRe:またはRE:という記号を付加する。この略号は、受け取ったメールの表題「○○」に対し返事の表題「○○に関して」を自動的に付ける便宜上のものであり、送信者が意図的に削除しても構わない。古くから商用文で使われていた慣習が、電子メール発祥期のメールコマンドに採用され、さらにはRFCに記載されたことで定着したが、他にも諸説ある。

詳細は「Re:」を参照

Fw、Fwd（転送）

一部の電子メールクライアントでは、メールを転送する際に、件名の先頭に自動的に Fw: などの記号を付加することがある。この略号は Re と同様単なる便宜的なものであるだけでなく、RFCにすら記述の無い独自仕様である。例えば、Fw: が連続していれば何度も転送されたメールだと考えることもできるが、それはあくまで、一部の電子メールクライアントの仕様に過ぎず、一般的な理解ではない。Fw: の連続はチェーンメールに多いため、チェーンメールかどうかの目安にもなる。そのため、転送時に Fw: を削除するように指示する内容が記述されたチェーンメールもある。

歴史

先駆的活動

テキスト形式のメッセージを電気的に伝える方法は1800年代中頃のモールス信号による電報に遡る事が出来る。1939年のニューヨーク万国博覧会では、IBMが将来郵便に替わる高速の自社用電波を用いた通信で、祝福の文書をサンフランシスコからニューヨークに送った。第二次世界大戦中、ドイツが使用したテレタイプ端末は、その後テレックスが世界的に普及する1960年代末まで使われた。アメリカには同様なTWXがあり、1980年代末まで重要な通信方法の位置を占めた。

歴史的に「electronic mail（エレクトロニック・メール）」という用語は一般的に電子化された送信文書全般を指して用いられた。例えば、1970年代前半にはファクシミリによる文書送信を指す用語として用いられる例もあった。

電子メールの起源

電子メールはインターネットに先行して開発された。既存の電子メールシステムはインターネットを作るに当たって重要な道具となった。

最初の電子メールは1965年、メインフレーム上のタイムシェアリングシステムの複数の利用者が相互に通信する方法として使われ始めた。1970年代初頭までに、アメリカ国防総省の自動デジタル・ネットワーク (AUTODIN) は1350台の端末間を繋ぎ、1件あたり平均3000文字のメッセージを月間3000万件取り扱えるようになった。AUTODINは18台の計算機化された大きな切替装置が運用を支え、さらに約2500台の端末を繋げるアメリカ共通役務庁のアドバンスト・レコード・システムとも接続された。正確なところは不明だがその類の機能を持つ最初のシステムとして、SDC（ランド研究所からのスピンオフでSAGEのソフトウェア開発を行った会社）のQ32システムがある。マサチューセッツ工科大学は1961年にCTSSを導入し、複数の利用者が離れた端末から電話回線を使って中央システムにログインし、ディスクにファイルを保存し共有できる体制を整えた。 電子メールは間もなく利用者が異なるコンピュータ間で情報をやり取りするための「ネットワーク電子メール」に拡張された。1966年には異なるコンピュータ間で電子メールを転送していた（SAGEでの詳細は明らかではないが、もっと早い時期に実現していたかもしれない）。

• 1962年：事務管理用端末システム (IBM)1440/1460

• 1968年：事務管理用端末システム (IBM) ATS/360

• 1972年：UNIXメール

• 1972年：ラリー・ブリードによるAPLメールボックス

• 1974年：PLATO IV Notes - オンラインのインターネットコミュニティシステムにおいて、「個人的な注意」を通知するための電子メールが広く使われた。

• 1978年：ニュージャージー医科歯科大学の電子メール

• 1981年：IBM OfficeVisionのPROFS

• 1982年：ディジタル・イクイップメント・コーポレーションのALL-IN-1

ARPANETは電子メールの発展に多大な影響を与えた。その誕生直後の1969年にシステム間電子メール転送の実験を行ったという報告がある。BBN社のレイ・トムリンソンは1971年にARPANET上の電子メールシステムを開発し、初めて@を使って利用者名と機器とを指定できるようにした。ARPANET上では電子メール利用者が急激に増大し、1975年には1000人以上が利用するようになっていた。

その他にも、1978年までにUNIXメールがネットワーク化されUUCPとなり、1981年にはIBMのメインフレームの電子メールがBOTNETで接続された。

一般への浸透

ARPANETでの電子メールの利便性と利点が一般に知られるようになると、電子メールの人気が高まり、ARPANETへの接続ができない人々からもそれを要求する声が出てきた。タイムシェアリングシステムを代替ネットワークで接続した電子メールシステムがいくつも開発された。例えばUUCPやIBMのVNETなどがある。

全てのコンピュータやコンピュータネットワークが直接相互に接続されるわけではないので、電子メールのアドレスには情報の伝達「経路」、つまり送信側コンピュータから受信側コンピュータまでのパスを示す必要があった。電子メールはこの経路指定方法でいくつものネットワーク間（ARPANET、BITNET、NSFNET）でやり取りすることができた。UUCPで接続されたホストとも電子メールをやり取りすることが可能であった。

経路は「バングパス」と呼ばれる方法で指定された。あるホストから直接到達可能なホストのアドレスを書き、そこから次に到達可能なホストのアドレスをバング（感嘆符＝!）で接続して書いていくアドレス指定方式である。

CCITTは、種々の電子メールシステムの相互運用を可能とするために 1980年代にX.400標準規格を開発した。同じ頃、IETFがもっと単純なプロトコルSimple Mail Transfer Protocol (SMTP) を開発し、これがインターネット上の電子メール転送のデファクトスタンダードとなった。インターネットに各家庭から接続するようになった現代では、SMTPを基礎とする電子メールシステムの相互運用性は逆にセキュリティ上の問題を生じさせている。

1982年、ホワイトハウスは国家安全保障会議 (NSC) 従事者のために IBM の電子メールシステム Professional Office System （PROFシステム）を採用した。1985年4月、このシステムがNSC従事者向けに完全動作するようになった。1986年11月、ホワイトハウスの残りの部分もオンライン化された。1980年代末ごろまではPROFシステムだけだったが、その後は様々なシステムが導入されている（VAX A-1（オールインワン）や、cc:Mailなど）。

問題

トラフィックの増大と配送遅延

電子メールのトラフィックの多くは実はスパムメールである。ある報告によると、2007年中に送信されたメールのうち90%から95%がスパムメールであったという。大量に送信されるこれらのスパムメールはメールサーバに過大な負荷を与え、メール配送遅延の原因となることもある。たとえば2004年7月下旬から8月上旬にかけて、大手インターネットプロバイダ@niftyで、海外から大量に送信されたスパムメールによりメールサーバに断続的な負担が掛かり、メールの受信に支障が生じる状態が続いた。（2010年10月ロシアで摘発されたスパムメール業者は1日500億通を発信していたという。）

また近年、トロイの木馬などのマルウェアに感染したコンピュータ群によって引き起こされるDDoS型のスパム送信の割合が急激に増加しており、ますますメールサーバに多大な負荷を及ぼすものとされている（→ボットネットを参照）。

スパム以外のトラフィック増大要因として、いわゆる「年賀メール」（元旦前後に発生する大量の挨拶メール）の類もある。特に携帯電話・PHSのメール機能は「即時の意思疏通を図る手段」としてチャット的に利用される場合があるため、一般の電子メールに比べ大量かつ集中的に送信されやすく、これを原因とした配送遅延や輻輳が問題になる場合もある。この対策として、各通信事業者が年越時間帯の利用自粛を呼び掛けたり発信制限を行ったりすることもある。かつてパソコン通信が全盛だった時代には、処理の集中を防ぐため、あらかじめ年賀メールをサーバに予約送信しておき元旦に順次配送するといったサービスも提供されていた。

なお、電子メールの配送システムの多くは、メールサーバに一定以上の負荷が掛かると送信を保留し一旦スプールに保存し後に（例えば数時間後に）再送信を試みる仕組みになっているため、トラフィックが一定量を超えると配送の極端な遅延が起こる。この遅延はメール1通毎に起こるため、同時期に送ったメールであっても、あるものは数秒で届きあるものは数時間で届くということになり、これを理解していない利用者の間ではメールを「送った」「送らない」で揉める恐れもある。

一時的なトラフィックの増大でスプールに保存された保留メールは、多くの場合時間の経過と共に処理され正常に戻るが、メールサーバの能力が十分でないと再送処理自体が間に合わなくなり、送信者に失敗通知が返送されることもある。なお、失敗通知すら返送されず「消滅」することは原理的にありえない。メールサーバは能力が追い付かない場合メールの受信(SMTPコネクション)自体を拒否するからである。よく年賀メール等で「トラフィック増大が原因であるプロバイダのメールの紛失が起きた」と、あたかも不可抗力であるが如き報道を目にするが、正確にはそのプロバイダのメールサーバの管理が適切でなく、混雑時の処理が正しく動作していないシステム不良である。

同時多発テロ時には、ニューヨーク周辺間のメールが1日遅延するなどした他、2009年には南アフリカでケープタウンとヨハネスブルグ間700kmで実験が行われ、電子メールより伝書鳩の方が早く情報を伝達できた。

スパムメール対策の問題点

スパムメール対策としてサーバ上、クライアント上でのフィルタリングが普及してきたが、誤検知により通常のメールがスパムであると判断されてしまい、不着となる問題が増えている（→電子メールフィルタリングを参照）。

コミュニケーション上の問題

方言などが原因で口頭での意思疏通が叶わない人同士でも、文字による意思疏通を図れるため、人の交流が広域化した現代ではメールによる意思疏通は有用である（日本、中国、イギリスなど方言が多様な国では特に）。一方、パソコン通信やインターネット等における文字だけの遣り取りに見られる問題（炎上、Flaming）は電子メールにおいても見られる。メールの真意、感情が相手に伝わらず、度々揉め事に発展するケースが挙げられている。英語圏では、メールの真意を読み取り間違え、感情に任せて送るメールの呼称（スラング）にFlame Mailというものがある。

脚注

関連項目

• メールアドレス
• プロバイダ
• メールマガジン
• フリーメールサービス
• プッシュ型電子メール
• Webメール
• メーリングリスト
• メールサーバ - Domain Name System（DNS）
• 電子メールクライアント（電子メールソフト、メールクライアント、MUA）
• スパム (メール)（迷惑メール）
• ストアアンドフォワード
• mailto:

外部リンク

• 大和総研/コラム：フィッシング詐欺が再喚起するHTMLメールの危険性
• 署名区切り行：sig-dashes
• Email Standards Project （英語）

wiki en

[Wiki en表示]

This article is about the communications medium. For the former manufacturing conglomerate, see Email Limited.

The at sign, a part of every SMTP email address

Electronic mail, most commonly referred to as email or e-mail since c. 1993, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the same time, in common with instant messaging. Today's email systems are based on a store-and-forward model. Email servers accept, forward, deliver, and store messages. Neither the users nor their computers are required to be online simultaneously; they need connect only briefly, typically to an email server, for as long as it takes to send or receive messages.

Historically, the term electronic mail was used generically for any electronic document transmission. For example, several writers in the early 1970s used the term to describe fax document transmission. As a result, it is difficult to find the first citation for the use of the term with the more specific meaning it has today.

An Internet email message consists of three components, the message envelope, the message header, and the message body. The message header contains control information, including, minimally, an originator's email address and one or more recipient addresses. Usually descriptive information is also added, such as a subject header field and a message submission date/time stamp.

Originally a text-only (ASCII) communications medium, Internet email was extended to carry, e.g., text in other character sets, multi-media content attachments, a process standardized in RFC 2045 through 2049. Collectively, these RFCs have come to be called Multipurpose Internet Mail Extensions (MIME). Subsequent RFC's have proposed standards for internationalized email addresses using UTF-8.

Electronic mail predates the inception of the Internet and was in fact a crucial tool in creating it, but the history of modern, global Internet email services reaches back to the early ARPANET. Standards for encoding email messages were proposed as early as 1973 (RFC 561). Conversion from ARPANET to the Internet in the early 1980s produced the core of the current services. An email sent in the early 1970s looks quite similar to a basic text message sent on the Internet today.

Network-based email was initially exchanged on the ARPANET in extensions to the File Transfer Protocol (FTP), but is now carried by the Simple Mail Transfer Protocol (SMTP), first published as Internet standard 10 (RFC 821) in 1982. In the process of transporting email messages between systems, SMTP communicates delivery parameters using a message envelope separate from the message (header and body) itself.

Spelling

Electronic mail has several English spelling options that occasionally prove cause for vehement disagreement.

• e-mail is the most common form in print, and is recommended by some prominent journalistic and technical style guides^{[citation needed]}. According to Corpus of Contemporary American English data, this is the form that appears most frequently in edited, published American English and British English writing.
• email is the most common form used online, and is required by IETF Requests for Comment and working groups and increasingly by style guides. This spelling also appears in most dictionaries.
• mail was the form used in the original RFC. The service is referred to as mail and a single piece of electronic mail is called a message.
• eMail, capitalizing only the letter M, was common among ARPANET users and the early developers of Unix, CMS, AppleLink, eWorld, AOL, GEnie, and Hotmail.^{[citation needed]}
• EMail is a traditional form that has been used in RFCs for the "Author's Address", and is expressly required "for historical reasons".
• E-mail is sometimes used, capitalizing the initial letter E as in similar abbreviations like E-piano, E-guitar, A-bomb, H-bomb, and C-section.

There is also some variety in the plural form of the term. In US English email is used as a mass noun (like the term mail for items sent through the postal system), but in British English it is more commonly used as a count noun with the plural emails.^{[citation needed]}

Origin

The AUTODIN network provided message service between 1,350 terminals, handling 30 million messages per month, with an average message length of approximately 3,000 characters. Autodin was supported by 18 large computerized switches, and was connected to the United States General Services Administration Advanced Record System, which provided similar services to roughly 2,500 terminals.

Host-based mail systems

With the introduction of MIT's Compatible Time-Sharing System (CTSS) in 1961 multiple users were able to log into a central system from remote dial-up terminals, and to store and share files on the central disk. Informal methods of using this to pass messages developed and were expanded to create the first system worthy of the name "email":

• 1965 – MIT's CTSS MAIL.

Other early systems soon had their own email applications:

• 1962 – 1440/1460 Administrative Terminal System
• 1968 – ATS/360
• 1972 – Unix mail program
• 1972 – APL Mailbox by Larry Breed
• 1974 – The PLATO IV Notes on-line message board system was generalized to offer 'personal notes' (email) in August, 1974.
• 1978 – EMAIL at University of Medicine and Dentistry of New Jersey
• 1981 – PROFS by IBM
• 1982 – ALL-IN-1 by Digital Equipment Corporation

Though they're all similar in concept, these original email systems had widely different features and ran on systems that were incompatible with each other. They allowed communication only between users logged into the same host or "mainframe," although there might be hundreds or thousands of users within an organization.

LAN email systems

In the early 1980s, networked personal computers on LANs became increasingly important. Server-based systems similar to the earlier mainframe systems were developed. Again, these systems initially allowed communication only between users logged into the same server infrastructure. Examples include:

• cc:Mail
• Lantastic
• WordPerfect Office
• Microsoft Mail
• Banyan VINES
• Lotus Notes

Eventually these systems too could link different organizations as long as they ran the same email system and proprietary protocol.

Email networks

Soon systems were developed to link compatible mail programs between different organisations over dialup modems or leased lines, creating local and global networks.

• In 1971 the first ARPANET email was sent, and through RFC 561, RFC 680, RFC 724, and finally 1977's RFC 733, became a standardized working system.

Other, separate networks were also being created including:

• Unix mail was networked by 1978's uucp, which was also used for USENET newsgroup postings
• IBM mainframe email was linked by BITNET in 1981
• IBM PCs running DOS in 1984 could link with FidoNet for email and shared bulletin board posting

Attempts at interoperability

Early interoperability among independent systems included:

• ARPANET, the forerunner of today's Internet, which defined the first protocols for dissimilar computers to exchange email
• uucp implementations for non-Unix systems, which were used as an open "glue" between differing mail systems, primarily over dialup telephones
• CSNET, which used dial-up telephone access to link additional sites to the ARPANET and then Internet

There were later efforts at interoperability standardization too:

• Novell briefly championed the open MHS protocol but abandoned it after purchasing the non-MHS WordPerfect Office (renamed Groupwise)
• The Coloured Book protocols on UK academic networks until 1992
• X.400 in the 1980s and early 1990s was promoted by major vendors and mandated for government use under GOSIP but abandoned by all but a few – in favor of Internet SMTP by the mid-1990s.

From SNDMSG to MSG

In the early 1970s, Ray Tomlinson updated an existing utility called SNDMSG so that it could copy messages (as files) over the network. Lawrence Roberts, the project manager for the ARPANET development, took the idea of READMAIL, which dumped all "recent" messages onto the user's terminal, and wrote a program for TENEX in TECO macros called RD, which permitted access to individual messages. Barry Wessler then updated RD and called it NRD.

Marty Yonke rewrote NRD to include reading, access to SNDMSG for sending, and a help system, and called the utility WRD, which was later known as BANANARD. John Vittal then updated this version to include three important commands: Move (combined save/delete command), Answer (determined to whom a reply should be sent) and Forward (sent an email to a person who was not already a recipient). The system was called MSG. With inclusion of these features, MSG is considered to be the first integrated modern email program, from which many other applications have descended.

Rise of ARPANET mail

The ARPANET computer network made a large contribution to the development of email. There is one report that indicates experimental inter-system email transfers began shortly after its creation in 1969. Ray Tomlinson is generally credited as having sent the first email across a network, initiating the use of the "@" sign to separate the names of the user and the user's machine in 1971, when he sent a message from one Digital Equipment Corporation DEC-10 computer to another DEC-10. The two machines were placed next to each other. Tomlinson's work was quickly adopted across the ARPANET, which significantly increased the popularity of email. For many years, email was the killer app of the ARPANET and then the Internet.

Most other networks had their own email protocols and address formats; as the influence of the ARPANET and later the Internet grew, central sites often hosted email gateways that passed mail between the internet and these other networks. Internet email addressing is still complicated by the need to handle mail destined for these older networks. Some well-known examples of these were UUCP (mostly Unix computers), BITNET (mostly IBM and VAX mainframes at universities), FidoNet (personal computers), DECnet (various networks) and CSNET, a forerunner of NSFNet.

An example of an Internet email address that routed mail to a user at a UUCP host:

hubhost!middlehost!edgehost!user@uucpgateway.somedomain.example.com

This was necessary because in early years UUCP computers did not maintain (and could not consult central servers for) information about the location of all hosts they exchanged mail with, but rather only knew how to communicate with a few network neighbors; email messages (and other data such as Usenet News) were passed along in a chain among hosts who had explicitly agreed to share data with each other. (Eventually the UUCP Mapping Project would provide a form of network routing database for email.)

Operation overview

The diagram to the right shows a typical sequence of events that takes place when Alice composes a message using her mail user agent (MUA). She enters the email address of her correspondent, and hits the "send" button.

1. Her MUA formats the message in email format and uses the Submission Protocol (a profile of the Simple Mail Transfer Protocol (SMTP), see RFC 6409) to send the message to the local mail submission agent (MSA), in this case smtp.a.org, run by Alice's internet service provider (ISP).
2. The MSA looks at the destination address provided in the SMTP protocol (not from the message header), in this case bob@b.org. An Internet email address is a string of the form localpart@exampledomain. The part before the @ sign is the local part of the address, often the username of the recipient, and the part after the @ sign is a domain name or a fully qualified domain name. The MSA resolves a domain name to determine the fully qualified domain name of the mail exchange server in the Domain Name System (DNS).
3. The DNS server for the b.org domain, ns.b.org, responds with any MX records listing the mail exchange servers for that domain, in this case mx.b.org, a message transfer agent (MTA) server run by Bob's ISP.
4. smtp.a.org sends the message to mx.b.org using SMTP.

This server may need to forward the message to other MTAs before the message reaches the final message delivery agent (MDA).

1. The MDA delivers it to the mailbox of the user bob.
2. Bob presses the "get mail" button in his MUA, which picks up the message using either the Post Office Protocol (POP3) or the Internet Message Access Protocol (IMAP4).

That sequence of events applies to the majority of email users. However, there are many alternative possibilities and complications to the email system:

• Alice or Bob may use a client connected to a corporate email system, such as IBM Lotus Notes or Microsoft Exchange. These systems often have their own internal email format and their clients typically communicate with the email server using a vendor-specific, proprietary protocol. The server sends or receives email via the Internet through the product's Internet mail gateway which also does any necessary reformatting. If Alice and Bob work for the same company, the entire transaction may happen completely within a single corporate email system.
• Alice may not have a MUA on her computer but instead may connect to a webmail service.
• Alice's computer may run its own MTA, so avoiding the transfer at step 1.
• Bob may pick up his email in many ways, for example logging into mx.b.org and reading it directly, or by using a webmail service.
• Domains usually have several mail exchange servers so that they can continue to accept mail when the main mail exchange server is not available.
• Email messages are not secure if email encryption is not used correctly.

Many MTAs used to accept messages for any recipient on the Internet and do their best to deliver them. Such MTAs are called open mail relays. This was very important in the early days of the Internet when network connections were unreliable. If an MTA couldn't reach the destination, it could at least deliver it to a relay closer to the destination. The relay stood a better chance of delivering the message at a later time. However, this mechanism proved to be exploitable by people sending unsolicited bulk email and as a consequence very few modern MTAs are open mail relays, and many MTAs don't accept messages from open mail relays because such messages are very likely to be spam.

Message format

The Internet email message format is now defined by RFC 5322, with multi-media content attachments being defined in RFC 2045 through RFC 2049, collectively called Multipurpose Internet Mail Extensions or MIME. RFC 5322 replaced the earlier RFC 2822 in 2008, and in turn RFC 2822 in 2001 replaced RFC 822 – which had been the standard for Internet email for nearly 20 years. Published in 1982, RFC 822 was based on the earlier RFC 733 for the ARPANET.

Internet email messages consist of two major sections:

• Header – Structured into fields such as From, To, CC, Subject, Date, and other information about the email.
• Body – The basic content, as unstructured text; sometimes containing a signature block at the end. This is exactly the same as the body of a regular letter.

The header is separated from the body by a blank line.

Message header

Each message has exactly one header, which is structured into fields. Each field has a name and a value. RFC 5322 specifies the precise syntax.

Informally, each line of text in the header that begins with a printable character begins a separate field. The field name starts in the first character of the line and ends before the separator character ":". The separator is then followed by the field value (the "body" of the field). The value is continued onto subsequent lines if those lines have a space or tab as their first character. Field names and values are restricted to 7-bit ASCII characters. Non-ASCII values may be represented using MIME encoded words.

Header fields

Email header fields can be multi-line, and each line should be at most 78 characters long and in no event more than 998 characters long. Header fields defined by RFC 5322 can only contain US-ASCII characters; for encoding characters in other sets, a syntax specified in RFC 2047 can be used. Recently the IETF EAI working group has defined some standards track extensions, replacing previous experimental extensions, to allow UTF-8 encoded Unicode characters to be used within the header. In particular, this allows email addresses to use non-ASCII characters. Such characters must only be used by servers that support these extensions.

The message header must include at least the following fields:

• From: The email address, and optionally the name of the author(s). In many email clients not changeable except through changing account settings.
• Date: The local time and date when the message was written. Like the From: field, many email clients fill this in automatically when sending. The recipient's client may then display the time in the format and time zone local to him/her.

The message header should include at least the following fields:

• Message-ID: Also an automatically generated field; used to prevent multiple delivery and for reference in In-Reply-To: (see below).
• In-Reply-To: Message-ID of the message that this is a reply to. Used to link related messages together. This field only applies for reply messages.

RFC 3864 describes registration procedures for message header fields at the IANA; it provides for permanent and provisional message header field names, including also fields defined for MIME, netnews, and http, and referencing relevant RFCs. Common header fields for email include:

• To: The email address(es), and optionally name(s) of the message's recipient(s). Indicates primary recipients (multiple allowed), for secondary recipients see Cc: and Bcc: below.
• Subject: A brief summary of the topic of the message. Certain abbreviations are commonly used in the subject, including "RE:" and "FW:".
• Bcc: Blind Carbon Copy; addresses added to the SMTP delivery list but not (usually) listed in the message data, remaining invisible to other recipients.
• Cc: Carbon Copy; Many email clients will mark email in your inbox differently depending on whether you are in the To: or Cc: list.
• Content-Type: Information about how the message is to be displayed, usually a MIME type.
• Precedence: commonly with values "bulk", "junk", or "list"; used to indicate that automated "vacation" or "out of office" responses should not be returned for this mail, e.g. to prevent vacation notices from being sent to all other subscribers of a mailinglist. Sendmail uses this header to affect prioritization of queued email, with "Precedence: special-delivery" messages delivered sooner. With modern high-bandwidth networks delivery priority is less of an issue than it once was. Microsoft Exchange respects a fine-grained automatic response suppression mechanism, the X-Auto-Response-Suppress header.
• References: Message-ID of the message that this is a reply to, and the message-id of the message the previous reply was a reply to, etc.
• Reply-To: Address that should be used to reply to the message.
• Sender: Address of the actual sender acting on behalf of the author listed in the From: field (secretary, list manager, etc.).
• Archived-At: A direct link to the archived form of an individual email message.

Note that the To: field is not necessarily related to the addresses to which the message is delivered. The actual delivery list is supplied separately to the transport protocol, SMTP, which may or may not originally have been extracted from the header content. The "To:" field is similar to the addressing at the top of a conventional letter which is delivered according to the address on the outer envelope. In the same way, the "From:" field does not have to be the real sender of the email message. Some mail servers apply email authentication systems to messages being relayed. Data pertaining to server's activity is also part of the header, as defined below.

SMTP defines the trace information of a message, which is also saved in the header using the following two fields:

• Received: when an SMTP server accepts a message it inserts this trace record at the top of the header (last to first).
• Return-Path: when the delivery SMTP server makes the final delivery of a message, it inserts this field at the top of the header.

Other header fields that are added on top of the header by the receiving server may be called trace fields, in a broader sense.

• Authentication-Results: when a server carries out authentication checks, it can save the results in this field for consumption by downstream agents.
• Received-SPF: stores the results of SPF checks.
• Auto-Submitted: is used to mark automatically generated messages.
• VBR-Info: claims VBR whitelisting

Message body

Content encoding

Email was originally designed for 7-bit ASCII. Most email software is 8-bit clean but must assume it will communicate with 7-bit servers and mail readers. The MIME standard introduced character set specifiers and two content transfer encodings to enable transmission of non-ASCII data: quoted printable for mostly 7 bit content with a few characters outside that range and base64 for arbitrary binary data. The 8BITMIME and BINARY extensions were introduced to allow transmission of mail without the need for these encodings, but many mail transport agents still do not support them fully. In some countries, several encoding schemes coexist; as the result, by default, the message in a non-Latin alphabet language appears in non-readable form (the only exception is coincidence, when the sender and receiver use the same encoding scheme). Therefore, for international character sets, Unicode is growing in popularity.

Plain text and HTML

Most modern graphic email clients allow the use of either plain text or HTML for the message body at the option of the user. HTML email messages often include an automatically generated plain text copy as well, for compatibility reasons.

Advantages of HTML include the ability to include in-line links and images, set apart previous messages in block quotes, wrap naturally on any display, use emphasis such as underlines and italics, and change font styles. Disadvantages include the increased size of the email, privacy concerns about web bugs, abuse of HTML email as a vector for phishing attacks and the spread of malicious software.

Some web based Mailing lists recommend that all posts be made in plain-text, with 72 or 80 characters per line for all the above reasons, but also because they have a significant number of readers using text-based email clients such as Mutt.

Some Microsoft email clients allow rich formatting using RTF, but unless the recipient is guaranteed to have a compatible email client this should be avoided.

In order to ensure that HTML sent in an email is rendered properly by the recipient's client software, an additional header must be specified when sending: "Content-type: text/html". Most email programs send this header automatically.

Servers and client applications

The interface of an email client, Thunderbird.

Messages are exchanged between hosts using the Simple Mail Transfer Protocol with software programs called mail transfer agents (MTAs); and delivered to a mail store by programs called mail delivery agents (MDAs, also sometimes called local delivery agents, LDAs). Users can retrieve their messages from servers using standard protocols such as POP or IMAP, or, as is more likely in a large corporate environment, with a proprietary protocol specific to Novell Groupwise, Lotus Notes or Microsoft Exchange Servers. Webmail interfaces allow users to access their mail with any standard web browser, from any computer, rather than relying on an email client. Programs used by users for retrieving, reading, and managing email are called mail user agents (MUAs).

Mail can be stored on the client, on the server side, or in both places. Standard formats for mailboxes include Maildir and mbox. Several prominent email clients use their own proprietary format and require conversion software to transfer email between them. Server-side storage is often in a proprietary format but since access is through a standard protocol such as IMAP, moving email from one server to another can be done with any MUA supporting the protocol.

Accepting a message obliges an MTA to deliver it, and when a message cannot be delivered, that MTA must send a bounce message back to the sender, indicating the problem.

Filename extensions

Upon reception of email messages, email client applications save messages in operating system files in the file system. Some clients save individual messages as separate files, while others use various database formats, often proprietary, for collective storage. A historical standard of storage is the mbox format. The specific format used is often indicated by special filename extensions:

eml

Used by many email clients including Microsoft Outlook Express, Lotus notes, Windows Mail, Mozilla Thunderbird, and Postbox. The files are plain text in MIME format, containing the email header as well as the message contents and attachments in one or more of several formats.

emlx

Used by Apple Mail.

msg

Used by Microsoft Office Outlook and OfficeLogic Groupware.

mbx

Used by Opera Mail, KMail, and Apple Mail based on the mbox format.

Some applications (like Apple Mail) leave attachments encoded in messages for searching while also saving separate copies of the attachments. Others separate attachments from messages and save them in a specific directory.

Mobile devices, such as cell phones and tablet computers, commonly have the ability to receive e-mail. Since users may always have their mobile device with them, users may access e-mail significantly faster on these devices than through other methods, such as desktop computers or laptops.

URI scheme mailto

Main article: mailto

The URI scheme, as registered with the IANA, defines the mailto: scheme for SMTP email addresses. Though its use is not strictly defined, URLs of this form are intended to be used to open the new message window of the user's mail client when the URL is activated, with the address as defined by the URL in the To: field.

Types

Web-based email (webmail)

This is the type of email that most users are familiar with. Many free email providers host their serves as web-based email (e.g. AOL Mail, Gmail, Outlook.com and Yahoo! Mail). This allows users to log into the email account by using a web browser to send and receive their email. Its main disadvantage is the need to be connected to the internet while using it. Other software tools exist which integrate parts of the webmail functionality into the OS (e.g. creating messages directly from third party applications via MAPI).

POP3 email services

POP3 is the acronym for Post Office Protocol 3. It is a leading email account type on the Internet. In a POP3 email account, email messages are downloaded to the client device (i.e. a computer) and then they are deleted from the mail server. It is difficult to save and view messages on multiple devices. Also, the messages sent from the computer are not copied to the Sent Items folder on the devices. The messages are deleted from the server to make room for more incoming messages. POP supports simple download-and-delete requirements for access to remote mailboxes (termed maildrop in the POP RFC's). Although most POP clients have an option to leave messages on the server after downloading a copy of them, most e-mail clients using POP3 simply connect, retrieve all messages, store them on the client device as new messages, delete them from the server, and then disconnect. Other protocols, notably IMAP, (Internet Message Access Protocol) provide more complete and complex remote access to typical mailbox operations. Many e-mail clients support POP as well as IMAP to retrieve messages; however, fewer Internet Service Providers (ISPs) support IMAP.^{[citation needed]}

IMAP email servers

IMAP refers to Internet Message Access Protocol. It is an alternative to the POP3 email. With an IMAP account, a user's account has access to mail folders on the mail server and can use any compatible device to read messages, as long as such a device can access the server. It shows the headers of messages, the sender and the subject and the device needs to request to download specific messages. Usually mail is saved on a mail server, therefore it is safer and it is backed up on an email server.

MAPI email servers

Messaging Application Programming Interface (MAPI) is a messaging architecture and a Component Object Model based API for Microsoft Windows.

Use

Flaming

Flaming occurs when a person sends a message with angry or antagonistic content. The term is derived from the use of the word Incendiary to describe particularly heated email discussions. Flaming is assumed to be more common today because of the ease and impersonality of email communications: confrontations in person or via telephone require direct interaction, where social norms encourage civility, whereas typing a message to another person is an indirect interaction, so civility may be forgotten.

Email bankruptcy

Main article: Email bankruptcy

Also known as "email fatigue", email bankruptcy is when a user ignores a large number of email messages after falling behind in reading and answering them. The reason for falling behind is often due to information overload and a general sense there is so much information that it is not possible to read it all. As a solution, people occasionally send a boilerplate message explaining that the email inbox is being cleared out. Harvard University law professor Lawrence Lessig is credited with coining this term, but he may only have popularized it.

In business

Email was widely accepted by the business community as the first broad electronic communication medium and was the first 'e-revolution' in business communication. Email is very simple to understand and like postal mail, email solves two basic problems of communication: logistics and synchronization (see below).

LAN based email is also an emerging form of usage for business. It not only allows the business user to download mail when offline, it also allows the small business user to have multiple users' email IDs with just one email connection.

Pros

• The problem of logistics: Much of the business world relies upon communications between people who are not physically in the same building, area or even country; setting up and attending an in-person meeting, telephone call, or conference call can be inconvenient, time-consuming, and costly. Email provides a way to exchange information between two or more people with no set-up costs and that is generally far less expensive than physical meetings or phone calls.
• The problem of synchronisation: With real time communication by meetings or phone calls, participants have to work on the same schedule, and each participant must spend the same amount of time in the meeting or call. Email allows asynchrony: each participant may control their schedule independently.

Cons

Most business workers today spend from one to two hours of their working day on email: reading, ordering, sorting, 're-contextualizing' fragmented information, and writing email. The use of email is increasing due to increasing levels of globalisation – labour division and outsourcing amongst other things. Email can lead to some well-known problems:

• Loss of context: which means that the context is lost forever; there is no way to get the text back. Information in context (as in a newspaper) is much easier and faster to understand than unedited and sometimes unrelated fragments of information. Communicating in context can only be achieved when both parties have a full understanding of the context and issue in question.
• Information overload: Email is a push technology – the sender controls who receives the information. Convenient availability of mailing lists and use of "copy all" can lead to people receiving unwanted or irrelevant information of no use to them.
• Inconsistency: Email can duplicate information. This can be a problem when a large team is working on documents and information while not in constant contact with the other members of their team.
• Liability. Statements made in an email can be deemed legally binding and be used against a party in a court of law.

Despite these disadvantages, email has become the most widely used medium of communication within the business world. In fact, a 2010 study on workplace communication, found that 83% of U.S. knowledge workers felt that email was critical to their success and productivity at work.

Research on email marketing

Research suggests that email marketing can be viewed as useful by consumers if it contains information such as special sales offerings and new product information. Offering interesting hyperlinks or generic information on consumer trends is less useful. This research by Martin et al. (2003) also shows that if consumers find email marketing useful, they are likely to visit a store, thereby overcoming limitations of Internet marketing such as not being able to touch or try on a product.

Problems

Speed of correspondence

Despite its name implying that its use is faster than either postal (physical) mail or telephone calls, correspondence over email often varies incredibly steeply — ranging from communication that is indeed semi-instant (often the fastest when a person is already sitting in front of a computer with their email program open, or when the person has email services automatically set up to speedily check for new messages on their mobile phone) to communication that can quite literally take weeks or even months to garner a response. In the case of the latter, it often proves much more rapid to call the person via telephone or via some other means of audio. Therefore, as a rule, unless one's workplace or social circle already communicates heavily via email in a rapid manner, a person should assume that email runs a perpetual risk of actually being slower as a communication mode than either mobile phone or text messaging communication.^{[citation needed]}

This general rule of thumb is often perplexing to those who use email heavily but whose colleagues and friends do not. Meanwhile, some people, due to exasperation with not getting responses to urgent messages, may eventually decline to use email with any regularity at all, and may be put in the sometimes-awkward position of having to notify their friends and colleagues who do use email regularly, that this is not a good way to reach them.

Attachment size limitation

Main article: Email attachment

Email messages may have one or more attachments, i.e., MIME parts intended to provide copies of files. Attachments serve the purpose of delivering binary or text files of unspecified size. In principle there is no technical intrinsic restriction in the InternetMessage Format, SMTP protocol or MIME limiting the size or number of attachments. In practice, however, email service providers implement various limitations on the permissible size of files or the size of an entire message.

Furthermore, due to technical reasons, often a small attachment can increase in size when sent, which can be confusing to senders when trying to assess whether they can or cannot send a file by email, and this can result in their message being rejected.

As larger and larger file sizes are being created and traded, many users are either forced to upload and download their files using an FTP server, or more popularly, use online file sharing facilities or services, usually over web-friendly HTTP, in order to send and receive them.

Information overload

A December 2007 New York Times blog post described information overload as "a $650 Billion Drag on the Economy", and the New York Times reported in April 2008 that "E-MAIL has become the bane of some people's professional lives" due to information overload, yet "none of the current wave of high-profile Internet start-ups focused on email really eliminates the problem of email overload because none helps us prepare replies". GigaOm posted a similar article in September 2010, highlighting research that found 57% of knowledge workers were overwhelmed by the volume of email they received. Technology investors reflect similar concerns.

In October 2010, CNN published an article titled "Happy Information Overload Day" that compiled research on email overload from IT companies and productivity experts. According to Basex, the average knowledge worker receives 93 emails a day. Subsequent studies have reported higher numbers. Marsha Egan, an email productivity expert, called email technology both a blessing and a curse in the article. She stated, "Everyone just learns that they have to have it dinging and flashing and open just in case the boss e-mails," she said. "The best gift any group can give each other is to never use e-mail urgently. If you need it within three hours, pick up the phone."

Spamming and computer viruses

The usefulness of email is being threatened by four phenomena: email bombardment, spamming, phishing, and email worms.

Spamming is unsolicited commercial (or bulk) email. Because of the minuscule cost of sending email, spammers can send hundreds of millions of email messages each day over an inexpensive Internet connection. Hundreds of active spammers sending this volume of mail results in information overload for many computer users who receive voluminous unsolicited email each day.

Email worms use email as a way of replicating themselves into vulnerable computers. Although the first email worm affected UNIX computers, the problem is most common today on the Microsoft Windows operating system.

The combination of spam and worm programs results in users receiving a constant drizzle of junk email, which reduces the usefulness of email as a practical tool.

A number of anti-spam techniques mitigate the impact of spam. In the United States, U.S. Congress has also passed a law, the Can Spam Act of 2003, attempting to regulate such email. Australia also has very strict spam laws restricting the sending of spam from an Australian ISP, but its impact has been minimal since most spam comes from regimes that seem reluctant to regulate the sending of spam.^{[citation needed]}

Email spoofing

Main article: Email spoofing

Email spoofing occurs when the header information of an email is designed to make the message appear to come from a known or trusted source. Spam and phishing emails typically use such spoofing to mislead the recipient about the origin of the message.

Email bombing

Main article: Email bomb

Email bombing is the intentional sending of large volumes of messages to a target address. The overloading of the target email address can render it unusable and can even cause the mail server to crash.

Privacy concerns

Main article: Internet privacy

Today it can be important to distinguish between Internet and internal email systems. Internet email may travel and be stored on networks and computers without the sender's or the recipient's control. During the transit time it is possible that third parties read or even modify the content. Internal mail systems, in which the information never leaves the organizational network, may be more secure, although information technology personnel and others whose function may involve monitoring or managing may be accessing the email of other employees.

Email privacy, without some security precautions, can be compromised because:

• email messages are generally not encrypted.
• email messages have to go through intermediate computers before reaching their destination, meaning it is relatively easy for others to intercept and read messages.
• many Internet Service Providers (ISP) store copies of email messages on their mail servers before they are delivered. The backups of these can remain for up to several months on their server, despite deletion from the mailbox.
• the "Received:"-fields and other information in the email can often identify the sender, preventing anonymous communication.

There are cryptography applications that can serve as a remedy to one or more of the above. For example, Virtual Private Networks or the Tor anonymity network can be used to encrypt traffic from the user machine to a safer network while GPG, PGP, SMEmail, or S/MIME can be used for end-to-end message encryption, and SMTP STARTTLS or SMTP over Transport Layer Security/Secure Sockets Layer can be used to encrypt communications for a single mail hop between the SMTP client and the SMTP server.

Additionally, many mail user agents do not protect logins and passwords, making them easy to intercept by an attacker. Encrypted authentication schemes such as SASL prevent this.

Finally, attached files share many of the same hazards as those found in peer-to-peer filesharing. Attached files may contain trojans or viruses.

Tracking of sent mail

The original SMTP mail service provides limited mechanisms for tracking a transmitted message, and none for verifying that it has been delivered or read. It requires that each mail server must either deliver it onward or return a failure notice (bounce message), but both software bugs and system failures can cause messages to be lost. To remedy this, the IETF introduced Delivery Status Notifications (delivery receipts) and Message Disposition Notifications (return receipts); however, these are not universally deployed in production. (A complete Message Tracking mechanism was also defined, but it never gained traction; see RFCs 3885 through 3888.)

Many ISPs now deliberately disable non-delivery reports (NDRs) and delivery receipts due to the activities of spammers:

• Delivery Reports can be used to verify whether an address exists and so is available to be spammed
• If the spammer uses a forged sender email address (email spoofing), then the innocent email address that was used can be flooded with NDRs from the many invalid email addresses the spammer may have attempted to mail. These NDRs then constitute spam from the ISP to the innocent user

There are a number of systems that allow the sender to see if messages have been opened. The receiver could also let the sender know that the emails have been opened through an "Okay" button. A check sign can appear in the sender's screen when the receiver's "Okay" button is pressed.

U.S. government

The U.S. federal government has been involved in email in several different ways.

Starting in 1977, the U.S. Postal Service (USPS) recognized that electronic mail and electronic transactions posed a significant threat to First Class mail volumes and revenue. Therefore, the USPS initiated an experimental email service known as E-COM. Electronic messages were transmitted to a post office, printed out, and delivered as hard copy. To take advantage of the service, an individual had to transmit at least 200 messages. The delivery time of the messages was the same as First Class mail and cost 26 cents. Both the Postal Regulatory Commission and the Federal Communications Commission opposed E-COM. The FCC concluded that E-COM constituted common carriage under its jurisdiction and the USPS would have to file a tariff. Three years after initiating the service, USPS canceled E-COM and attempted to sell it off.

The early ARPANET dealt with multiple email clients that had various, and at times incompatible, formats. For example, in the Multics, the "@" sign meant "kill line" and anything before the "@" sign was ignored, so Multics users had to use a command-line option to specify the destination system. The Department of Defense DARPA desired to have uniformity and interoperability for email and therefore funded efforts to drive towards unified inter-operable standards. This led to David Crocker, John Vittal, Kenneth Pogran, and Austin Henderson publishing RFC 733, "Standard for the Format of ARPA Network Text Message" (November 21, 1977), which was apparently not effective. In 1979, a meeting was held at BBN to resolve incompatibility issues. Jon Postel recounted the meeting in RFC 808, "Summary of Computer Mail Services Meeting Held at BBN on 10 January 1979" (March 1, 1982), which includes an appendix listing the varying email systems at the time. This, in turn, lead to the release of David Crocker's RFC 822, "Standard for the Format of ARPA Internet Text Messages" (August 13, 1982).

The National Science Foundation took over operations of the ARPANET and Internet from the Department of Defense, and initiated NSFNet, a new backbone for the network. A part of the NSFNet AUP forbade commercial traffic. In 1988, Vint Cerf arranged for an interconnection of MCI Mail with NSFNET on an experimental basis. The following year Compuserve email interconnected with NSFNET. Within a few years the commercial traffic restriction was removed from NSFNETs AUP, and NSFNET was privatised.

In the late 1990s, the Federal Trade Commission grew concerned with fraud transpiring in email, and initiated a series of procedures on spam, fraud, and phishing. In 2004, FTC jurisdiction over spam was codified into law in the form of the CAN SPAM Act. Several other U.S. federal agencies have also exercised jurisdiction including the Department of Justice and the Secret Service.

NASA has provided email capabilities to astronauts aboard the Space Shuttle and International Space Station since 1991 when a Macintosh Portable was used aboard Space Shuttle mission STS-43 to send the first email via AppleLink. Today astronauts aboard the International Space Station have email capabilities via the wireless networking throughout the station and are connected to the ground at 3 Mbit/s Earth to station and 10 Mbit/s station to Earth, comparable to home DSL connection speeds.

See also

Email terminologies

• Email encryption
• HTML email
• Internet fax

• Privacy-enhanced Electronic Mail
• Push email
• X-Originating-IP

Email social issues

• Anti-spam techniques (email)
• CompuServe (first consumer service)
• Computer virus
• E-card
• Email art
• Email jamming
• Email spam
• Email spoofing

• Email storm
• List of email subject abbreviations
• Information overload
• Internet humor
• Internet slang
• Netiquette
• Posting style
• Usenet quoting

Clients and servers

• Biff
• Email address
• Email authentication
• Email client, Comparison of email clients
• Email hosting service

• Internet mail standards
• Mail transfer agent
• Mail user agent
• Unicode and email
• Webmail, Comparison of webmail providers

Mailing list

• Anonymous remailer
• Disposable email address
• Email digest
• Email encryption

• Email tracking
• Electronic mailing list
• Mailer-Daemon
• Mailing list archive

History

• Telegraphy
• Lexigram
• MCI Mail

Protocols

• IMAP
• POP3
• SMTP

• UUCP
• X400

Notes

References

Further reading

• Cemil Betanov, Introduction to X.400, Artech House, ISBN 0-89006-597-7.
• Marsha Egan, "Inbox Detox and The Habit of Email Excellence", Acanthus Publishing ISBN 978-0-9815589-8-1
• Lawrence Hughes, Internet e-mail Protocols, Standards and Implementation, Artech House Publishers, ISBN 0-89006-939-5.
• Kevin Johnson, Internet Email Protocols: A Developer's Guide, Addison-Wesley Professional, ISBN 0-201-43288-9.
• Pete Loshin, Essential Email Standards: RFCs and Protocols Made Practical, John Wiley & Sons, ISBN 0-471-34597-0.
• Partridge, Craig (April–June 2008). "The Technical Development of Internet Email" (PDF). IEEE Annals of the History of Computing (Berlin: IEEE Computer Society) 30 (2). ISSN 1934-1547 
• Sara Radicati, Electronic Mail: An Introduction to the X.400 Message Handling Standards, Mcgraw-Hill, ISBN 0-07-051104-7.
• John Rhoton, Programmer's Guide to Internet Mail: SMTP, POP, IMAP, and LDAP, Elsevier, ISBN 1-55558-212-5.
• John Rhoton, X.400 and SMTP: Battle of the E-mail Protocols, Elsevier, ISBN 1-55558-165-X.
• David Wood, Programming Internet Mail, O'Reilly, ISBN 1-56592-479-7.
• Yoram M. Kalman & Sheizaf Rafaeli, Online Pauses and Silence: Chronemic Expectancy Violations in Written Computer-Mediated Communication, Communication Research, Vol. 38, pp. 54–69, 2011

External links

• E-mail at the Open Directory Project
• IANA's list of standard header fields
• The History of Email is Dave Crocker's attempt at capturing the sequence of 'significant' occurrences in the evolution of email; a collaborative effort that also cites this page.
• The History of Electronic Mail is a personal memoir by the implementer of an early email system
• The Official MCI Mail Blog! a blog about MCI Mail, one of the early commercial electronic mail services

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★リンクテーブル★

リンク元	「電子メール」「electronic mail」

「電子メール」

　　[★]

英

electronic mail、email

「electronic mail」

　　[★]

電子メール

関

email