Infrastructure

rDNSReverse DNS

Definition

Reverse DNS (rDNS) is the process of resolving an IP address back to a hostname, the opposite of normal “forward” DNS. It works by looking up the PTR record for an IP: ask “who is 203.0.113.10?” and rDNS returns mail.example.com. Receiving mail servers run an rDNS check on every connection, and an IP with no valid reverse DNS is one of the oldest and strongest spam signals there is.

  • Resolves an IP address to a hostname, the reverse of normal DNS
  • Powered by the PTR record stored in the reverse zone
  • Receivers run it on every incoming connection as a trust check
  • No valid rDNS is a near-guaranteed path to the spam folder or a rejection
At a glance
Also called Reverse DNS, rDNS
Looks up IP address to hostname
Powered by PTR records
Reverse zone in-addr.arpa / ip6.arpa
Stronger form FCrDNS (forward-confirmed)
Required by Gmail & Yahoo on sending IPs (2024)

What reverse DNS does

Forward DNS answers “what IP is behind this name?” Reverse DNS answers the mirror question: “what name is behind this IP?” It is not a separate system; it uses the same DNS, just with a PTR record stored in a special reverse zone. To look up 203.0.113.10, a resolver queries the name 10.113.0.203.in-addr.arpa and reads back the hostname from its PTR.

You can run an rDNS lookup yourself from a terminal. It is the fastest way to see what hostname, if any, a sending IP announces.

Running a reverse DNS lookup from the command line 
$ dig -x 203.0.113.10 +short
mail.example.com.

$ host 203.0.113.10
10.113.0.203.in-addr.arpa domain name pointer mail.example.com.

Why reverse DNS decides whether your mail gets in

The moment your server connects, the receiver runs an rDNS lookup on your IP. The result feeds straight into its spam scoring. An IP with no PTR at all, or one with a generic ISP-style name like 203-0-113-10.dynamic.isp.net, looks like a hijacked home machine or a throwaway cloud box, exactly what botnets use. A clean, dedicated hostname says “a real, configured mail server lives here.”

Most serious receivers go a step further and require forward-confirmed reverse DNS: the hostname your PTR returns must itself resolve back to the same IP. Since February 2024, Gmail and Yahoo make this mandatory for senders. If you run your own mail server, setting clean reverse DNS is one of the highest-value, lowest-effort deliverability fixes available; verify it with our reverse DNS checker.

How reverse DNS is checked at connection time

Your server opens an SMTP connection from its IP
The receiver runs a reverse DNS (PTR) lookup on that IP
What comes back?
Nothing: strong spam signal Generic ISP name: weak Clean mail hostname: good
The receiver confirms the hostname resolves forward to the same IP
Reverse DNS passes and the message is judged on its merits

Forward DNS vs reverse DNS

Forward DNS Reverse DNS
Question What IP is this name? What name is this IP?
Record A / AAAA PTR
Set by You / your DNS host The IP block owner
Email use Finding mail servers Vetting connecting IPs

By the numbers

Every
connection: receivers run a reverse DNS check on every incoming SMTP session.
Feb 2024
Gmail and Yahoo began requiring valid, matching reverse DNS on sending IPs from this date.

Common mistakes

Running a mail server with no PTR at all
An IP with no reverse DNS is treated as untrusted by default. Many servers reject or heavily penalise it before reading a single header. Set a PTR before you send.
Accepting the default dynamic hostname
Generic names like 203-0-113-10.dynamic.isp.net mark an IP as residential or throwaway space. Replace it with a dedicated hostname that matches a forward record.
Forgetting reverse DNS after changing IPs
A new sending IP starts with no useful PTR. If you migrate servers or add an IP and forget the reverse DNS, deliverability drops the moment you cut over.

Frequently asked questions

What is reverse DNS in email?
Reverse DNS resolves a sending IP address back to a hostname using its PTR record. Receiving mail servers run this check on every connection to judge whether the IP belongs to a legitimate mail server. An IP with no valid reverse DNS is a strong spam signal.
Is reverse DNS required to send email?
Effectively yes for anyone running their own mail server. Since February 2024, Gmail and Yahoo require sending IPs to have valid forward and reverse DNS, and many other receivers have penalised missing rDNS for years. Without it, your mail is likely rejected or filtered to spam.
How do I check my reverse DNS?
Run dig -x or host against your sending IP from a terminal, or use our reverse DNS checker. You want a clean, dedicated hostname that also resolves forward back to the same IP, which is forward-confirmed reverse DNS.
Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← Back to glossary