http://expita.com/nomime.html (link verified 2008-05-25) explains very clearly why HTML is bad in email and you should be sending plain text. Even better, it explains how to set up almost every mail program (including Aol, Hotmail, MSN, and Yahoo). http://www.delorie.com/listserv/mime/ gives instructions for Outlook and Netscape (link verified 2008-05-25).
If that page is unavailable, there are a number of mirrors where you can get the same text.
It would be surprising if every link above stops working, but if so you can run this pre-programmed Google search. The document is about 100K long, so don’t bother clicking on any hits less than 90K long. Also check that the revision date (at the end of the document) is Sep 22, 2003, or later.