Reverse DNS is what your IP address resolves back to. Set it under Networking → Reverse DNS (/client/network/reverse-dns/) for the primary address of any server and for any floating IP you hold.
Setting it
Enter a fully qualified hostname, for example mail.example.com. Rules:
- Fully qualified: at least two labels.
mailalone is rejected - Letters, digits and hyphens only, and each label starts and ends with a letter or digit
- Not an IP address
- Up to 253 characters
Clearing the field removes the PTR record.
Set forward DNS too
A PTR that points at a name which does not resolve back to the same address is worse than none, and mail servers check both. In your own DNS:
mail.example.com. A 203.0.113.10
Then set the PTR for 203.0.113.10 to mail.example.com. Confirm both:
dig +short -x 203.0.113.10
dig +short mail.example.com
Propagation
Changes are published immediately but resolvers cache, so allow time before concluding it did not work. Check against a public resolver rather than your own:
dig +short -x 203.0.113.10 @1.1.1.1
Sending mail from a VPS
Matching forward and reverse DNS is necessary and not sufficient. You also need SPF, DKIM and DMARC on the sending domain, and a clean sending reputation on the address, which a newly allocated address does not have. Using a transactional mail provider is usually less work than earning that reputation.