First suggestion: post your question on http://forums.mozillazine.org
Its the 'official' forum for Mozilla (insert product name here) questions.

Second, I'm gonne need some more info. Such as:
- Your current settings (just the non blank ones) when you view the properties of your account in MTB, the same for SMTP settings, incuding the advanced ones (if applicable).
- Is it an IMAP / POP / .... box?
- The exact error message.

Obvious, but:

DO NOT post your username, password or e-mail adress.