RETURNMXCHECK

Rejects senders if they don't have a valid return MX.

Default:

off

Affects:

qmail-smtpd

Example:

"" (any value will do)

Note:

This is quite useful even though it doesn't stops many spammers today.
But it ensures that you can send an email or a bounce back to the sender.
It is also useful for your own users/customers because if they type a nonexistent sender into their "from" field (typos!) it'll stop them the first time instead of getting helpdesk calls when nobody can reply to them.