Mac Mail Stuck in Send Loop

Hello, I am trying to send email and when I hit send the message flashes on the screen, disappears as though it's been sent, then re-appears a second later. this keeps going and there is no way to stop it without quitting out of mail. I've tried deleting the outbox, deleting the offending email, restarting...nothing works. Can anyone on here throw me a solution? Help me Obi-Wan, you're my only hope!