virtual users, mailer daemon send mails to non existant recipient and dovecot store it

[Sean Greenslade]

On Tue, Aug 23, 2016 at 05:57:37PM +0200, Sam wrote:
> Hello,
> 
> Sometime when we receive a spam or virus that is detected as it, mailer
> daemon send a reply to the sender to inform that the message is a spam or
> content viruses.

You probably shouldn't do this. The vast majority of spam / virus emails
are sent from compromised machines / botnets, use fake return paths, and
either don't monitor replies, or just use replies to verify that the
email address is valid and send more spam to it. Or worse, it can turn
your server into a spamming machine if the return addresses are set to
other people's email addresses.

There are several valid ways of handling spam, depending on how your
mail architecture works. One is to reject incoming spam messages at the
receiving mailserver. The downside is that this leaks information to the
spammers about what spam methods actually get through or not.

Another method is to accept all incoming messages, then sort /
quarantine / blackhole any spam. The downside is that this makes your
server seem more accepting, which may attract more spam.

I personally take the second approach, though which is better will
definitely depend on how your specific system works.

If you're really dead set on having some sort of auto reply, at the very
least make it only reply to senders that have historically sent good
messages (e.g. some sort of whitelist).

--Sean