Problem with Mail...it doesn't shut down

I have two Gmail accounts (IMAP) which I use with Mail app. When I shut the computer down, I have to force quit Mail or open the activities (cmd-0) and close all the activities...if I don't do that I'm not able to shut the mac down.I can't understand how to solve this problem...I've already tried to reinstall Mail, I repaired the permissions, I set up the accounts several times but nothing has changed...

Hope someone will be able to help me!

Thanks