How do I delete an outgoing email that keeps crashing Mail?

Hello,

I tried sending an email this morning. It was addressed to about 220 people, all BCCd.

When I hit send, I got the message:

Cannot send message using the server Gmail [it's a gmail account, obviously]

The server "smtp.gmail.com" did not recognize the following recipients:

Big list of every email address

I couldn't get out of this screen, so I force quit.

Now that message is still in the Outbox, and every time I try to open Mail, it tries to send that message again, and I get the same error message.

I ended up going onto gmail.com and just sending it from there with no issues.

I would like to delete this message so it would just stop crashing Mail, but I can't figure out how to do it.

Not sure how to proceed!

I'm running 10.7.3, the version of Mail is 5.2

Any help is much appreciated, thank you!