What is reverse DNS (PTR) and why does email need it?

Reverse DNS is a PTR record that maps an IP address back to a host name, the opposite of a normal DNS lookup. Mail servers check it on the sending IP, and a missing or mismatched PTR record is a common reason mail is rejected or filtered. The best result is a host name that resolves back to the same IP address.

Why it matters

Anyone can claim any name in an email, but the owner of an IP address controls its reverse record. Receivers use it as a basic sanity check on the sender. Google's sender guidelines ask that sending IPs have valid forward and reverse DNS, and many other receivers apply similar checks. A server with no PTR, or a generic one, looks like a throwaway machine.

How to check

Find the IP address your mail is sent from. In a received message, read the Received headers, or ask your mail provider. If you use a large email service, it manages this for you.
Look up the PTR record:

dig -x 203.0.113.10 +short

The answer is a host name, for example mail.example.com..

Look up that host name and confirm it points back to the same address. This is called forward-confirmed reverse DNS:

dig A mail.example.com +short

Compare the host name with the name your mail server announces when it connects (the HELO or EHLO name). Matching names make the setup easier for receivers to trust.
If the PTR is missing or wrong, change it with whoever owns the IP address. You cannot set it in your own domain's DNS, because reverse records live in zones controlled by the network owner. This is normally your hosting company, cloud provider or internet provider, and many have a control panel field or a support request for it.
After the change, repeat the lookups. Caches can take a while to refresh.

Crawvix's scan checks "Sending IP has matching reverse DNS" for your domain.

Common mistakes

Trying to add a PTR record to your domain's DNS zone, where it has no effect.
Setting a PTR host name that does not resolve back to the same IP.
Leaving a generic provider name on a dedicated mail server.
Forgetting IPv6, which has its own reverse zone and its own PTR record.
Having several PTR records for one IP, which can confuse checks.

Frequently asked questions

Do I need a PTR record if I use an email service?

Usually not. The service handles sending IPs and their reverse DNS. You still need to set up SPF, DKIM and DMARC for your own domain.

Is PTR part of SPF?

No. They are separate. SPF checks whether an IP may send for a domain, while PTR maps an IP to a host name.

Is reverse DNS enough to avoid spam folders?

No. It is one signal among many. Authentication, reputation and content also matter.

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