Here's a great overview of the reasons to eschew HTML for email, and an amazing, exhaustive primer on turning off html-mail-sending in dozens of mail clients. I keep my screen resolution high enough that most of the html mail I receive shows up at completely unreadable sizes — plain text mail automativally sizes to my preferred scale, but not so html mail.
Many E-mail and Usenet News reader programs, usually the mail and news reader programs that come with browser packages, allow users to include binary attachments (MIME or other encoding) or HTML (normally found on web pages) within their E-mail messages. This makes URLs into clickable links and it means that graphic images, formatting, and even color coded text can also be included in E-mail messages. While this makes your E-mail interesting and pretty to look at, it can cause problems for other people who receive your E-mail because they may use different E-mail programs, different computer systems, and different application programs whose files are often not fully compatible with each other. Any of these can cause trouble with in-line HTML (or encoded attachments). Most of the time all they see is the actual HTML code behind the message. And if someone replies to the HTML formatted message, the quoting can render the message even more unreadable. In some cases, the message is nothing but strange looking text. For this reason, many mailing lists especially those that provide a digest version, explicitly forbid the use of HTML formatted e-mail. See examples section.
(via Dive Into Mark)
