A mail server IP has no reverse DNS
ATTENTIONWhat it means
the offending value is published by example.com as one of its own mail servers, and it has no PTR record. Receivers routinely refuse mail from an address with no reverse DNS, Gmail among them, before the message is ever examined.
How to fix it
Ask whoever controls that IP (your host or cloud provider, not your DNS registrar) to set a PTR record pointing at the hostname the server introduces itself as.
Check your own domain
This page describes the condition. To find out whether your domain has it, run the PTR test or the full checkup, which examines every record at once.
Understand it properly
This page covers one condition. For the whole picture, Why mail lands in spam, in the order worth checking, or start at the Health Library.
Where this comes from
We judge this against RFC 1912 section 2.1. Every finding on this site cites the specification behind it so you can check the work rather than take our word for it.
Other PTR conditions
PTR_MISSING
Every fix on this site is yours to implement. If you would rather someone did it, I take this work directly.