From the "Duh" Files

One can't always guarantee that one's iBook is always going to be connected to a trusted network. For times like that, you can do your best to avoid sending your password in plaintext over the wire, and wires are not necessarily involved. Say, for instance, that you want to send some e-mail but you don't can't authenticate to your trusty old qmail box back home.

I've recalled that OS X includes a Postfix 2.1.5 install that I instinctively shut off as soon as humanly possible. But can it act as a relay? You bet your sweet ass. No magic needed.

So I ran "sudo postfix start" and set Thunderbird to use 127.0.0.1 as the default outgoing SMTP server. Piece of cake.