Every few months for the last several years, this particular problem annoyed me enough that I searched this site and the web in general for a possible solution. The closest thing I'd ever found was the Workaround for a stuck outbound Mail message hint. But that doesn't describe the situation I've encountered. Maybe that's because the solution was obvious to everyone but myself. I find that hard to swallow.

I tend to use my PowerBook while disconnected. I also tend to send messages while disconnected, so they stay in my Outbox until I reconnect. Generally, it works. But every once in a while, a message gets stuck; Mail.app flatly refuses to try to redeliver it. Synchronize Accounts does nothing (and never has). It used to be that sending another message via the same server would prod it into sending both messages, but somewhere along the way, Apple fixed that "feature." Usually quitting and restarting Mail.app (a lame solution to my mind) would cause it to retry. Still, every once in a while, I'd still have messages that defy even that treatment.

Recently, I serendipitously stumbled upon the solution: Drag the offending messages to the Outbox folder in the folders pane (which you'll only have if you tried to send messages while offline). They'll be redelivered. Yes, I am aware of the Message » Send Again menu option. But there are times when I want/need to keep the Date header the same (yes, it can be forged) and the time stamp of the cryptographic signature the same as the original send date and time. Dragging the message to the Outbox accomplishes all of those things.