Mail Help!!

Hi, I've been using Mail for a good long time, but just within the last week it, for some reason, decided it will only send emails (with any of my 6 email servers) when I first start it up. (When I restart the app it will send the messages that it didn't before). Sometimes they go into the outbox folder, sometimes it acts like it was sent, but there is no message received, or in the sent box folders. (Either way it usually sends them when the program is restarted). Any suggestions?