Hi Don,

You could use a packet sniffer utility, like EtherDetect, to store in a log all TCP packets your computer sends to a mail server.

Say, you are trying to use three different ways of sending emails. One of them works, two others dont. With such utility you can check all commands sent to server as well as responses received, compare them and find what's wrong.

It's not an immediate solution, of course, but it may be very helpful for this and similar tasks. I used it very productively to learn how to communicate with MSN/Hotmail servers.