I would vote to support both HTML and txt.
This leads me to another question: Does allowing HTML emails mean that clear-text mails will get "forbidden"? Actually I have problems with Non-HTML due to the mail policy of my employer (and I can't use another mail account or mail client at work) and due to a mail client that doesn't offer the possibility of a rule based selection of the mail styles. Nevertheless I recommend to stay with text-only as my experience is that quoting HTML email often results in a mess of characters (depending on the MUA). If a higher number of users really prefers HTML-style it might be a good idea to establish a web forum. Justmysct. Tobias.