Reverse DNS lookup
Enter an IP address or a hostname. We read the PTR record, then resolve that name forward to confirm it points back to the same address.
How receivers use your PTR record
When your server connects, the receiving server sees only an IP address. It looks up the PTR record for that IP, compares the name with the one your server gave in its EHLO greeting, and checks the name resolves back. Each mismatch is a point against you.
A missing PTR is the strongest of those signals. Home connections and hijacked machines rarely have one that matches a real mail host, so receivers read its absence as a sign of a botnet. Google's sender guidelines make valid forward and reverse DNS a requirement for anyone sending to Gmail.
The name itself matters too. A PTR like 203-0-113-10.dsl.example.net says residential range, and some filters score it down even when both directions match.
| What you see | What it means | What to do |
|---|---|---|
| mail.yourdomain.com, points back | Forward-confirmed, under your own domain | Nothing. Match your EHLO name to it. |
| A host name that points elsewhere | PTR exists, forward check fails | Add an A record for that name to this IP |
| 203-0-113-10.pool.isp.net | Generic name from the IP's owner | Ask the host for a custom PTR |
| None (NXDOMAIN) | No reverse DNS at all | Set one before you send from this IP |
PTR lookups follow RFC 1035 (in-addr.arpa) and RFC 3596 (ip6.arpa).
IPv4 and IPv6 are looked up differently
For IPv4 the octets are reversed under in-addr.arpa, so 198.51.100.7 becomes 7.100.51.198.in-addr.arpa. For IPv6 every hex digit becomes its own label, reversed, under ip6.arpa. That's 32 labels for one address.
You don't need to build those names. Paste the address as you have it, short IPv6 form included, and the tool expands it. Gmail applies stricter rules to IPv6 senders, so if your server sends over IPv6, check that address as well as the IPv4 one.
A correct PTR still needs a clean list
A correct PTR gets your connection accepted. It doesn't stop the receiver from counting every address that bounces, and bounces weigh on your IP reputation just as a missing PTR does. If the IP is already in trouble, our blacklist checker shows which lists name it.
ZapBounce runs its own SMTP check on port 25 for every address, closes before DATA so nothing is sent, and reports valid, invalid, catch-all or unknown. Unknowns and duplicates are never billed. See how the check works.
Questions people ask
What is a reverse DNS lookup?
A reverse DNS lookup turns an IP address into a hostname by reading its PTR record. For 203.0.113.10 the resolver asks for 10.113.0.203.in-addr.arpa. Mail servers use it to see whether a connecting IP belongs to a named, managed mail host.
Why does my mail server need a PTR record?
Google's sender guidelines require valid forward and reverse DNS for sending IPs, and many other receivers reject or junk mail from an IP with no PTR. It's one of the cheapest signals a receiver has that a server is a real mail host and not a compromised home machine.
What does forward-confirmed reverse DNS mean?
The PTR record names a host, and that host's A or AAAA record points back to the same IP. Both directions have to agree. A PTR that names a hostname with no matching address counts as missing to receivers that check.
How do I set or change a PTR record?
Ask whoever owns the IP address, which is your hosting provider or ISP, not your domain registrar. Most cloud hosts have a reverse DNS field in the server's network settings. Set it to a hostname under your own domain, then add an A record for that name pointing back to the IP.
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