So far, I haven't found the answer to this query, so I'll ask what is
probably a redundant question.
I am using sendmail as a relay host between our DMZ and internal
systems. Two GroupWise systems are the 800# gorillas; the others
systems could be handled by a relatively small whitelist.

I need to stop email to undeliverable addresses at the edge of our DMZ,
not after sendmail relays it beyond the edge, (i.e. I want sendmail to
generate the 551 response, not something internal that will require
sendmail to attempt to send a bounce message).

Can I get sendmail to do an LDAP query to Groupwise to determine if the
is an account for the (supposed) recipient. Ideally, we would be able
to check for either a GroupWise address or a "Groupwise nickname" for
the recipient.