Sent messages remain in outbox after successfully being delivered

[jcorso]

I have an odd problem with entourage. When I send a message, it goes
to my outbox, then gets delivered. Then, it gets copied to my IMAP
sent-mail folder. But, a copy remains in my outbox.

This wouldn't really be a problem, but every so often, when I shut
down entourage, I get a dialog box popping up saying "sending
messages..." and it tries to re-send every message in my outbox.

Any ideas on how to fix it? (I know the messages are being sent
because I've sent one to myself at a different email account...)

Thanks
Jason