HTML formatting is a bit of a work in progress, and probably always will be. What we’re really trying to say here is that e-mail clients are the nastiest offenders of markup-abuse on the market. We do our best with the code that they spit out, but sometimes, there’s just nothing we can do.
If you’re having trouble with a particular message, and the formatting is getting all messed up, just put “Ptext:” in the subject line of your message. Mailmanagr will then force itself to only deal with the plain text part of your message, and will ignore all of the HTML gobbledygook.