Wikipedia

HTML email

This is an old revision of this page, as edited by Omegatron (talk | contribs) at 18:17, 14 September 2006 (→Security vulnerabilities: «"a security issue for users, who may be" → "used in phishing attacks, in which users are", +", visiting it,", -"", "to a scammer" → "(like bank account numbers) to a scammer", +"revealing", "read"). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

HTML email is the use of a subset of HTML (often ill-defined) to provide formatting and semantic markup capabilities in email that are not available with plain text.

Most graphical e-mail clients support HTML email, and many default to it.[1] Many of these clients include both a GUI editor for composing HTML e-mails and a rendering engine for displaying received HTML e-mails.

Use of HTML in e-mail is controversial:

Benefits

  • The sender can express complex formatting, such as subscripts and superscripts, in scientific or mathematical formulas. Note that Unicode supports many such complex characters; however, font support issues with Unicode limit the usefulness of this possibility.
  • The sender can properly express headings, bulleted lists, emphasize text, or use other visual cues to improve the readability and aesthetics of the message.
  • Allows in-line inclusion of diagrams or mathematical formula as images, which are otherwise difficult to convey (typically using ASCII art).

Drawbacks

  • A few recipients have e-mail clients that cannot display HTML. This may be mitigated by the inclusion of an automatically generated plain text version, which may be missing important formatting information (e.g. an equation may lose a superscript and take on an entirely new meaning).
  • Some senders may excessively rely upon large, colorful, or distracting fonts making all but the shortest messages more difficult to read.
  • Many HTML-based GUI email clients automatically convert common plain text characters, e.g. — and ", into non-plain text equivalents. This can cause translation problems in other users' clients.

For these reasons many mailing lists deliberately block HTML e-mail, either stripping out the HTML part to just leave the plain text part or rejecting the entire message.

Multi-part formats

Many email clients are configured to send a plain text version of a message along with the HTML version, to ensure that it can be read even by text-only clients, using the Content-Type: multipart/alternative, as specified in RFC 1521.[2][3][4]

Message size

HTML e-mail is larger than plain text. Even if no special formatting is used, there will be the overhead from the tags used in a minimal HTML document, and if formatting is heavily used it may be much higher. Multi-part messages, with duplicate copies of the same content in different formats, increase the size even further.

Download speed was more of a concern in the 1990s, though, when most users were accessing email servers through slow modems. On a modern connection, the difference in download time between plain text and mixed message mail, which can be a factor of three or more, is negligible, especially when compared to images, music files, or other common attachments.[5]

Additionally, the plain text section of a multi-part message can be retrieved by itself, using IMAP's FETCH command.[6]

Security vulnerabilities

HTML allows for a link to have a different target than the link's text. This can be used in phishing attacks, in which users are fooled into believing that a link points to the website of an authoritative source (such as a bank), visiting it, and unintentionally revealing personal details (like bank account numbers) to a scammer.

If an email contains inline content from an external server, such as an image, the server can alert a third party that the e-mail has been opened. This is a potential privacy risk, revealing that an email address is real (so that it can be targeted in the future) and revealing when the message was read. For this reason, some e-mail clients do not load external images until requested to by the user.

Most E-mail spam is sent in HTML, so spam filters (such as Spamassassin) give high spam scores to HTML messages.

References

  1. ^ Configuring Mail Clients to Send Plain ASCII Text — E-mail client programs
  2. ^ RFC 1521 7.2.3. The Multipart/alternative subtype
  3. ^ TN1010-11-2: Multipart/Alternative – Gracefully handling HTML-phobic email clients.
  4. ^ Sending HTML and Plain Text E-Mail Simultaneously
  5. ^ HTML Email - Still Evil?
  6. ^ Do we really want to send web pages in e-mail?
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