Once an MTA has accepted the mail it either must deliver it or bounce it back to the sender with an explanation of why it can't be delivered. That's fine for Exchange and qmail which (IIRC) only accept-then-bounce; it does nothing for Sendmail, Postfix, and Exim users.
Err..... I know that postfix can bounce mail after it's recived.... I would be amazed if the other MTAs you mention cannot.
Sendmail, at least, can do even better: It can read the entire message, process it, and then issue a rejection saying "Whoops, sorry, we don't want this after all!" - all within the SMTP transaction.
This means you can analyze the heck out of a message and reject it, and you don't need to worry about whether the bounce notice is going to the right place. No risk of undeliverable bounces waiting around in your outgoing mail queue, plus you don't contribute to the flood of invalid "You sent us a virus!" notices going to people whose computers aren't infected.
Kelson Vibber
SpeedGate Communications <www.speed.net>
