Reverse DNS for mail servers

Every IP address that sends your mail should have a PTR record, and the name in that record has to resolve back to the same address. That round trip is called forward confirmed reverse DNS, and it is one of the cheapest trust signals there is. Plenty of receivers reject mail from an address with no PTR before they look at anything else.

bash

# what name does this address claim
dig -x 203.0.113.10 +short
# mail.example.com.

# does that name point back to it
dig mail.example.com +short
# 203.0.113.10

If those two do not agree you have a mismatch, which is treated as barely better than having no PTR at all.

You do not set this in your own zone

The PTR lives in the reverse zone for the IP block, which belongs to whoever owns the addresses. That means your hosting provider or your ISP, usually through a field in their control panel. This is the part people spend an hour looking for in their own DNS.

What name to use

Use the real hostname of the mail server, the same one it gives in its SMTP greeting and the same one your MX points at. Keeping all three the same costs nothing and avoids a whole class of odd rejections.

If you send over IPv6 as well, remember it needs its own PTR. Some providers set up IPv4 by default and quietly leave IPv6 without one, and a few large receivers are stricter about IPv6 than they are about IPv4.