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/