Robin,
Basically Michel's parser will choke on any tags it doesn't like. The big ones disallowed are table, font, blockquote. So about all you can do is pre for formatted code, bold and italic that seems to be enough. Also the offline navigator some people use doesn't handle html.
>What portion / all of the HTML is permitted in these messages? Do you have a chart of what is permitted, or do you simply support the entire markup language? I re-read the rules for posting and noted that PRE may be used to mark text which is coding, and that text is not permitted between {Start xxx...}'s. Could I suggest a short document that would demonstrate these techniques, and allow more colorful and maybe more meaningful (?) e-mail / thread postings.