Postfix – rejects non-existing accounts
One of the biggest differences between Postfix and qmail is that Postfix comes out of the box (more readily) configured for anti-spam functionality. A big feature (for us) is that Postfix will immediately reject messages targeted at accounts that do not exist. It appears to immediately disconnect and short circuit the connection.
At a minimum, I would guess that this feature alone reduces the level of spam actually received by 95% or more. It’s incredible to watch the spammers connect with absurd account names and try to deliver a message. Spammer connections occur about 1 every few seconds. I understand that they will often try dictionary attacks, but the account names that are attempted to be delivered are almost always bizzare mispellings. My hunch is that the spammers are hoping our server will actually bounce the message back to the forged ‘To’ address (which it used to do).
At any rate Postfix handily rejects these messages quickly, short circuiting the spammers. This saves a lot of load on the mail server. It never receives the message, once it determines the message is targeted for a non-existant account it rejects the delivery attempt.
In qmail, this was a major pain in the ass and was poorly documented. There is a patch called ‘validrcptto’ but it was a pain to get operational. Everything in qmail is a patch and a pain.