Problem deleting emails

After much Hesitation I've desided to start using Thunderbird (portable edition). However, when I try to delete an email I get "The current command did not succeed. The mail server responded: [TRYCREATE] The requested item could not be found.." I went offline and deleted the email, then again from the trash bin, and it was gone. But when I got online again, I received the same email I had just deleted (obviously, it's still in the server). Please help, thanks

PS: John (and helpers), thanks for all the fantastic programs that you're putting out. I really appreciate the "free" work you do here, and it makes me mad to see when some people complain. They just don't see what goes on behind the cameras.