Before sending a question to the mailing list, check whether it was already asked by using the archives' searchable index. The largest offender of this are the products from Microsoft, which, I'm fairly certain are responsible for the majority of email sent in the HTML format. I'm supposed to read. I hate to admit it, but I've contemplated moving to Netscape to read my mail, just to get around this problem. I haven't got there yet, but it has crossed my mind.
Flat text forces the author the decide presentation, which is a big no-no in communication.