RCCI, I had problems with the latest Thunderbird build, smtp connection was ok but no stamping of mails.
After much help here (and head scratching on my part) I un-installed and downloaded another copy of the Thunderbird install file, and haven't had any more problems.
It seemed to not save my smtp settings after a system shutdown, so perhaps the downloaded file was corrupt in some way. If you try a system shut down, then start and go to Thunderbird Tools, Account settings, Outgoing server, and see if it has retained the correct settings.
It might be one of the download mirrors has a problem (or had) with a slightly corrupted file.
Just food for thought, if it doesn't help you could try Bugzilla