An external content filter receives unfiltered mail from Postfix
(as described further below) and does one of the following:
Re-inject the mail back into Postfix, perhaps after changing
content and/or destination.
Reject the mail (by sending a suitable status code back to
Postfix). Postfix will return the mail to the sender.
NOTE: in this time of mail worms and forged spam, it is a VERY
BAD IDEA to send viruses back to the sender address, because the
sender address is almost certainly not the originator. It is better
to discard known viruses, and to quarantine material that is
suspect so that a human can decide what to do with it.