Like many of you, I had the same problem with Mail 3.0 in Leopard with outgoing mail. VERY disappointing considering the os otherwise. However after many failed attempts to get mail to go through, I finally did manage to get it to happen. Out of desparation, I used the outgoing settings for my web host, in this case,
MacHighway, and mail shot out of the outbox immediately. Although Embarq is my ISP, no combination of port and SSL's could get mail going out. I don't think I can fault Embarq for this, though. I used Terminal to telnet the open port (25) and had no problems.
I realize this may not be an option for everyone, just adding info to the thread. Hopefully Apple with have this fixed soon.