Best Practice: notify email sender that their reverse lookup is broken

exim can detect the presence of the A record && absence of the PTR record with ACL and then any action can be performed via routers. $sender_host_name, $host_lookup_failed and $host_lookup_deferred do all the trick.

The problem is that spammers are often use such poorly configured but still legal hosts for submission (ISP's dynamically addressed clients f.e.). So most of notifications will be addressed to the spammers.

The easiest way to avoid useless traffic is to accept such messages, check them against AV/SA/bayes and send notification only if they have passed DATA-checkout.

In terms of exim that mean that only if $sender_host_name is non-empty, $host_lookup_failed = 1 and $spam_score_int is less than threshold, you have to issue the notification. May be content-scanning tolerance should be significantly lowered for those messages.

Unfortunately I'm not familiar with postfix enough to propose an equivalent solution.