Postfix has an option reject_unknown_reverse_client_hostname that can be used like this:
smtpd_sender_restrictions = permit_mynetworks permit_sasl_authenticated reject_unknown_reverse_client_hostname
The meaning of this is that our mail server should refuse to accept emails from another mail server that does not have a PTR record on the DNS. A PTR record maps an IP address to a domain name. It’s like the reverse of an A record.
This check is intended to prevent emails from spammers. However I think that we should not use it in our mail servers, for these reasons:
Getting a PTR record for your server is not easy. It does not depend on you (on the configurations that you may do), but depends on your provider (ISP or VPS provider), on its technical capabilities and on its will, sometimes maybe on the amount of money that you can afford. A person that wants to have his own mail server most probably will not be able to afford it. So, it is not a good indication of spammer vs. non-spammer. By using it on our own server we penalize non-powerful users (like us).
I think that GMail and other systems don’t pay much importance to a PTR record too. If you have SPF+DKIM+DMARC set properly, your mails will not end up as spam. A mail never is discarded or considered as spam just because the sender does not have a PTR record. However the Postfix rule above does just that, it rejects a mail if the sending SMTP server does not have a PTR record. It is a very strong restriction.
What is your opinion? In your experience, how easy or difficult is to get a PTR record for your server?