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What Reverse DNS Is, and Why Mail Servers Check It
Forward DNS turns a name into an address. Reverse DNS does the opposite, and a missing or mismatched PTR record is one of the more common reasons legitimate mail gets flagged as spam.
5 min read
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Forward DNS and reverse DNS are opposite questions
A normal DNS lookup answers the question "what address does this name point to". Reverse DNS answers the opposite question: "what name has this address been given". The answer comes from a PTR record, short for pointer record, which lives in a special reverse zone rather than alongside a domain's normal A and MX records.
Critically, a PTR record is set by whoever controls the IP address, which is usually a hosting provider or ISP, not by whoever owns the domain the address happens to be serving. This is the opposite of forward DNS, where the domain owner is always the one in control.
Why this matters most for email
Receiving mail servers use reverse DNS as one of the earliest, cheapest signals available to judge whether a connecting server is legitimate. A server sending mail from an IP address with no PTR record at all, or one that reverses to something unrelated to the sending domain, looks a lot like a compromised machine or a spam operation that never bothered to configure anything properly. Many mail servers will reject or heavily penalise a connection on this basis alone, before even looking at the message content.
- No PTR record at all: often enough on its own to get a connection rejected by a strict mail server.
- PTR record present but pointing to something generic, like a hosting provider's default hostname: better than nothing, but still a weaker signal than one that matches the sending domain.
- PTR record that resolves back to a hostname which itself resolves back to the same IP, called forward confirmed reverse DNS: the strongest and most trusted configuration, and standard practice for any server that sends its own outbound mail.
Where else it comes up
Outside of email, reverse DNS is a quick way to get a human readable hint about an unfamiliar IP address showing up in server logs, firewall alerts or analytics data. A PTR record resolving to something like ec2-x-x-x-x.compute.amazonaws.com or an ISP's own naming pattern tells you at a glance roughly what kind of traffic you are looking at, well before pulling up a full IP lookup.
Checking a PTR record on Frabs
The Reverse DNS tool accepts either an IP address directly or a domain name, in which case it resolves the domain first and reverses whichever address comes back. If you run your own mail server, checking this against your own sending IP is worth doing once after setup and again any time you change hosting providers, since a new IP means a new, and possibly missing, PTR record.
Checking the same thing from a terminal
dig -x 1.1.1.1 +short
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