Mac Mail Problem

Basically, I sent an email with a massive attachment without thinking. Obviously the message did not go through, which was to be expected, but now it keeps popping up again and again in Drafts: On My Mac no matter how many times I delete it. (See attached screenshot for error message.) I've also tried Mailbox>Erase Deleted Messages>On My Mac with no better results. Since that day, the program has been unable to retrieve new mail, though continues to archive all my old messages. When I tried sending a test message to myself, I received it when I was logged into my email online, but it never arrived in my inbox in Mail.

Does anyone have a clue what to do here? Any help would be enormously appreciated.

Thanks,

Ash

I did try that, and deleted all drafts in my gmail through the web, but when I open Mail, the problem persists. My gmail works fine on the internet, but not in Mail, which leads me to think it's a problem with the program.