With Thunderbird 3.0.4:
I've just added an extra email account for one of my add on domains, with a bit of trial and error to get details correct. It's an IMAP account.
Received a test email to it OK. Tried to reply to that and got an error. I can't recreate this so unfort. can't give you the exact message, but it was words to the effect of "Sending message failed."
Closed and re-opened Thunderbird. Without changing the server settings, sending seems to be working now. However, in the left hand panel, under Inbox, I had these subfolders:
Where * is the character with four zeros in a box.
When I clicked on either of the bottom two, to delete them, I got this message:
"The current command did not succeed. The mail server responded:Mailbox does not exist, or must be subscribed to.
After clearing the messages I was able to delete the second folder, but not the top one. I closed down and restarted. The problem seemed to be cleared - one sent folder in the Trash, and nothing unwanted under Inbox.
Hope this helps.