qmail, BADRCPTTO but more so

[Felig]

I want to reject all mail coming to users without accounts. Mail to joe-xyzzy should be allowed if mail to joe is allowed. I want to reject it during the SMTP phase, not the usual bounce when delivery fails. BADRCPTTO requires a list of accounts to bounce; I want it to figure it out for itself. I realize that this might be tricky, considering aliases and all, and maybe the reason I can't find any such documentation is because it is too tricky. I don't particularly care if qmail simply drops the connection or sends a proper reject.