I've often referred subscribers to this Web site when suggesting plain 
text (it's often true that they really don't know, and have no 
practical way of finding out, just that that _means_: mail clients so 
often build the bells and whistles right into the foundation that 
users don't know they're there).

http://www.expita.com/nomime.html

You can find it by searching ["plain text" / email ] in Google.

It not only offers lots of specific suggestions, it has a clear 
explanation of why plain text in email matters.

-- Russ

Russell Hunt
Department of English
St. Thomas University
http://www.stu.ca/~hunt/