Here's a good piece on why plain text is better than formatted in email messages and it lists a ton of email packages and how to set them up for sending plain text messages:

Plain text is how your messages should be formatted when sending E-mail to mailing lists and Usenet newsgroups or to any other recipient. Though this rule is not yet cast in "Netiquette" stone, it is a good policy to follow if you want quick and informative responses to your questions and wish to avoid being "flamed" as a clueless newbie.

HTML is meant for the WWW; not for mailing lists, Usenet newsgroups postings, proper business E-mail correspondence and preferably not for personal E-mail unless the recipient is expecting it.

Reasons plain text is better, include:

• Plain text is smaller
• Plain text is safer (embedded HTML or MIME attachments spread viruses, worms, and trojans
• Plain text is more portable

I figure getting people to send their messages in plain text is a losing battle, but it makes your message simpler, safer and more readable so it's still something to think about at least.