Can't send messages in Mail

Hello all. You may remember me from my recent Migration Assistant travails. Well, I eventually got a new hard drive and everything copied over fine... Or so I thought.

I opened up Mail for the first time since the reinstall and happily went about my business, reading new messages and replying when necessary. However, this morning, two people told me that they never got my replies. I have done some testing, and here's what happens: I begin composing a new message, and after a bit it shows up in Drafts. When I hit "Send," it disappears from Drafts but never reappears in Sent Mail. The recipient never receives the message.

This happens every time I try to send a message from any of my accounts, all of which are set up properly. What could be the issue? I'm running 10.7.2, Mail version 5.1.

Thanks in advance!