Reverse DNS (PTR) Lookup
Find the hostname for an IPv4 or IPv6 address and verify forward-confirmed reverse DNS (FCrDNS), the check mail servers rely on.
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What this checks
Builds the in-addr.arpa / ip6.arpa name, fetches the PTR record and checks that the hostname resolves back to the same IP.
A PTR record maps an IP address back to a hostname. It lives in the reverse zone (in-addr.arpa for IPv4, ip6.arpa for IPv6), which is delegated to whoever owns the address block, not to the owner of your domain. That is why your DNS host cannot create it and your hosting provider or ISP has to.
Forward-confirmed reverse DNS (FCrDNS) goes one step further: the PTR hostname must itself resolve, through an A or AAAA record, to the same IP you started from. A PTR alone proves little, because whoever controls the address block can write any name into it. The forward lookup proves the owner of that name agrees. This tool shows both lookups side by side, so you can see which half is missing.
Mail servers use this check as a cheap reputation signal. RFC 1912 section 2.1 says every IP address should have a matching PTR record and that the PTR and A records should agree. Gmail's sender guidelines require the sending IP to have a PTR record whose forward lookup returns the same IP, and Microsoft's Outlook.com sender requirements expect valid reverse DNS as well. Without it, mail is rejected or deferred, for example with a Gmail 421 4.7.23 or 550 5.7.25 reply.
If you were sent here by a bounce, read the matching pages: the generic 5.7.25 bounce, the Gmail 5.7.25 bounce and the Gmail 4.7.23 deferral. For best results the PTR hostname should be a name you control, preferably the same name the server uses in its SMTP HELO/EHLO greeting. Changes can take as long as the PTR record TTL to appear.
If it fails
- No PTR: ask your hosting provider or ISP to set it; you cannot set PTR in your own zone.
- No FCrDNS: make the PTR hostname's A/AAAA record point to this IP.
FAQ
Why does my mail server need PTR?
Most receivers reject or spam-folder mail from IPs without a matching PTR.
Who controls PTR records?
The owner of the IP block — usually your hosting provider or ISP.
Can an IP have several PTRs?
Technically yes, but one is recommended for mail.
Does this work for IPv6?
Yes, using ip6.arpa.