On Fri, 10 Sep 2004 23:48:18 -0400, guy wrote
Does you firewall know what users are valid?
Or does it just accept email for anyone, then forward it to your SMTP
server?
If it accepts email for anyone, then the sender/spammer will never
drop an invalid email address.
It also lets your firewall act as a relay for "backscatter spam". If a
spammer sends mail to an invalid recipient, with MAIL FROM forged as being
from a third party, that third party will get the bounce message.
As an added bonus, when I fixed this situation on the gateway where I work,
the mail queues got a lot shorter because I was no longer trying repeatedly to
deliver bounce messages to hosts that were never going to accept any mail.