I have an external MTA configured with Postfix that delivers email to an
internal IMAP/LMTP
Dovecot server configured to bind an LDAP to check if users exist.
You should have postfix do the checking for whether or not users exist
and then have postfix reject and deny the message. Then you don't
care because the IP of the postfix server is almost certainly your MX server.
Plus backscatter. Sending a fail notification back to the sender (vs rejecting)
puts your mail server at risk of sending spam to email accounts that had their
address forged. And thus maybe yoru server being added to a spam blacklist.
