That's what I'm trying to figure out: Is my machine
infected with some sort of malware that is causing it
to function as stealth spam generator. If it is, how
do I stop it? If it's not, what the heck is going on?
The most likely explanation is that you're the victim of a
"Joe Job". Especially if your ISP is a big, well known one and your username is kind of common.
Otherwise, have you downloaded and installed any software recently from sites you aren't sure about? I don't know of any email zombie programs for the Mac, but that doesn't mean there aren't any.
Also, do you have a router/firewall? Or is your Mac connected directly to the DSL router? A router should protect you from most attacks, especially if you haven't fiddled around with its setup. If you don't have a router, do you have a good password on your computer? Preferably something with numbers and letters, and not in any dictionary.
Finally, you can use the "Activity Monitor" application from your "Utilities" folder to watch the network activity. There will always be some small amount of activity periodically, but if you see a steady amount of more than a few KB once in a while, then maybe you do have something going on.
charlie