Mail won't quit in 10.8.3

Hey, so I opened mail and checked a couple messages and then exit (not quit). Later, when I was trying to turn off my computer, I went to quit mail, but it didn't. The mail window wasn't there either. it was just "open". Whenever I try to restart or shut down my computer, it says that it can't until mail is quit.

How do i fix this?

Cheers