A little while ago I had a problem with the Thunderbird Sent folder.

I'm not too sure what had happened but it just wouldn't work right.

I found a page talking about the problem here:

http://forums.mozillazine.org/viewtopic.php?f=39&t=763585

The solution that worked for me is actually documented in that post. Ther eit is:

1. Go to your account folder and check the properties.
2. Click on "Copies & Folders" and then make sure that you use the "Sent" folder option
3. Enable Place a copy in under "When sending messages, automatically:"
4. And choose the Sent folder were you want your sent emails to go.

Thunderbird would work perfectly without that, but that's a good way to see your history and find email addresses that you otherwise misplaced, etc.

IMPORTANT NOTE

The Sent folder will NOT (re)appear until you sent at least ONE email, even if there were old emails in that folder.