I'd rather not switch back to Outlook Express. I'll deal with it for now, but if you can tell me details, I'll let the Mozilla folks know about the bug..
Try any MAPI function, e.g. madExcept's SendMapiMail or Jedi's MAPI function or Indy's MAPI function or any other MAPI function you can find on the internet. I think they all result in the same problem. IIRC, Thunderbird removes all line breaks from the body of all mails which it gets via either MAPI or mailto.