Well, I have the pre-forking server all set. I haven't made up my mind if I'll make it a true daemon. There's something elegant about just printing to stderr for logging. I'll have to look into qmail's daemons tools further. I've also started implementing SMTP into the server. I've done helo, and mail. For mail I've pretty much followed the RFCs. I notice that sendmail is a little bit more flexible in what it will handle.
I'll have to get back to the server code though, as I don't don't grow or shrink according to demand yet.