Mail 3.5 Sending Problem

For the first month on my new Mac I had no problem with Mail 3.5. One day after trying to send an email I got a huge delay and the message "Message cannot be appended to the outbox".

I did the "drag the mail folder to the desktop and restore" thing. Now when I press SEND I get the continuous spinning wheel next to the sent box. I just sent a message successfully on my iMac G4 running Tiger via the same account and server sans problem. Therefore the issue is in Mail 3.5 and Leopard on my Mac Pro.

Anyone with similar problems? Any suggestions for a solution? Should I zap PRAM/delete preferences/reinstall Mail??? Thanks in advance!