Rejecting during smtp

Hello.

One of the big issues with challenge-response is that the challenge goes to 
often fake email addresses and 'harasses'  innocent people.  Some people even 
treat it as spam.

There are qmail patches that will hold the smtp connection open, check if the 
user exists, and do a rejection at that point.  Blacklists do the same even 
putting in the blacklist information and code.

How about changing qconfirm to do the same ?

ie. During the smtp transactiion, hold the connection open, check to see if 
they are in the whitelist, if not, put the challenge email address in the 
rejection so the user can email to that challenge and get whitelisted.

ie. response...

spam check - send email to