Windows Live Mail works just fine.

The message I've tried to send went through, but the original problematic message is still stuck. It's an e-mail with a >25MB attachment that was supposed to be sent via a GMail account, until I noticed later on it's too big and violates GMail policies... but I receive no message that the e-mail couldn't be sent, and I still can't click the damn thing and delete it, nor can I move it anywhere else. The metro interface is very limiting in this situation.