Odd sending issue with SMTP

Here's the situation:

1. The email goes into the sent folder but it never gets sent.

2. This only occurs from a client, such as Thunderbird. If I send from the webmail interface (SqurrielMail) it sends.

3. There are no error messages when TBird sends. In fact, the emails appear in the Sent folder.

4. There is nothing in the logs about the TBird sends. I can see the emails sent by SquirrelMail in the logs. I can see the TBird logins, what folders the TBird user looked at etc. But, there is nothing -- not even an error message -- for the TBird sends.

5. There is nothing seemingly clogged anywhere. No mail queue trouble, etc.

I don't understand why this is happening. Shouldn't the mail logs at least show an error?

I've been running this server for about three years and I have never run into a problem like this. It wouldn't bother me a tenth as much if I could find an error somewhere in the logs!

What am I missing?

Check your outgoing server field for /the port/the auth mechanisms..
This should clear things out..

If nothing works, re-install Thunderbird. If that doesn't work, switch to a different version..