I am truly stunned. I could not make this up if I tried.
I knew that the ability to create wildcard SMTP mailboxes was exposed programmatically by digging through MSDN docs on the subject. I had never seen any KB article explaining how to do it. Bear in mind this is a feature that other mail handling programs have had since day 1. (That being the early '80s).
Thanks to TalkRoot, I found this pointer to KB315631. You really have to read it to believe it.
"Dear customer -- to solve the problem in your email software, fire up a compiler, compile this custom DLL, and there you go! Oh, did we mention you need to create a second SMTP instance that we're casually not going to tell you will need to sit on a different IP address on your machine, because you can't just have it listen to port 60025 or somesuch on the existing IP, because the mail forwarding syntax to send the unresolved messages to another server doesn't support custom target ports.".