David Lemson's WebLog  Product Unit Manager, Exchange
Configure Exchange 2003 to check recipients in SMTP protocol
• Comments 21   • Likes  For years, people have been asking me,
"how can I make Exchange work just like sendmail, where it
rejects invalid recipients during the SMTP protocol?"  Sendmail
has historically not had a directory, and so checking if a
recipient was valid was just a getpwnam() call, which is quick. 
Of course, when Exchange accepts a user who is not in the
directory, it will issue a non-delivery report later once it has
done a directory lookup, so Exchange is not actually open for
relay as some people might think. Accepting mail for nonexistant
users in a local domain does not meet my definition for "open
for relay".   For three major reasons, Exchange has not had this
feature:    1. Because Exchange has a distributed directory,
it's possible that one machine doesn't know about users added
elsewhere in the system, so you don't want one server to deny
those users just because the us