(It's tricky in these cases because it's a 'non-standard' setup - with a
catch-all POP3 account at an ISP, then a mail collector collecting from
there, and sorting messages out and rejecting messages to unrecognised users
- what should happen there? You can't do an SMTP reject, because the message
has already been 'accepted for final delivery' by the ISP, so you have to
send an NDN or not - the user isn't going to read the messages, so an NDN is
appropriate, but causes problems.)
Could you do some sort of technical hack, a call-forward to whatever is
doing the sorting to find out what addresses are valid?
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
"I dropped the toothpaste", said Tom, crestfallenly.