Forward DNS turns a name into an address: mail.example.com -> 203.0.113.25. Reverse DNS does the opposite, it turns an address back into a name using a PTR record. For websites this rarely matters. For mail servers it is critical: a missing or mismatched PTR is one of the most common reasons legitimate mail is throttled or rejected outright.
What is a PTR record?
A PTR record lives in a special reverse zone (in-addr.arpa for IPv4, ip6.arpa for IPv6) and maps an IP address to a hostname. Crucially, the PTR is controlled by whoever owns the IP block, usually your hosting provider or ISP, not your domain registrar. That is why you often cannot set it from the same place you manage your other DNS records.
Forward-confirmed reverse DNS (FCrDNS)
Receiving mail servers do not just check that a PTR exists, they check that it agrees with forward DNS. This round-trip is called FCrDNS:
- The sending IP is
203.0.113.25. - Its PTR says
mail.example.com. - Look up
mail.example.com, it must resolve back to203.0.113.25.
If all three line up, the sender is forward-confirmed. If the PTR points to a generic name like 203-0-113-25.pool.isp.net, or the forward lookup does not return the same IP, many servers treat the mail as suspicious.
You can run this exact check with the Reverse DNS / PTR tool, it reports the PTR hostname and whether it forward-confirms.
How to set it up correctly
- Set the PTR through your IP provider. Ask your host/ISP (or use their control panel) to set the PTR for your mail IP to a hostname you control, such as
mail.example.com. - Publish a matching forward (A/AAAA) record for that same hostname pointing back to the mail IP.
- Keep it consistent with your EHLO/HELO name. The name your mail server announces itself as should match the PTR and resolve to the same IP.
- Do both IPv4 and IPv6. If your server sends over IPv6, it needs a valid PTR for the IPv6 address too, a missing IPv6 PTR is an easy-to-miss cause of rejections.
Verify with the command line
# Reverse lookup (IP -> name)
dig +short -x 203.0.113.25
# Forward lookup of the returned name (should match the IP)
dig +short mail.example.com
If the two do not match, FCrDNS fails and you need to fix whichever side is wrong.
FAQ
Why can't I set the PTR in my normal DNS panel?
Because the reverse zone belongs to the owner of the IP address block, your hosting provider or ISP. Set the forward record in your panel, but request the PTR from them.
Does every server need reverse DNS?
Web servers generally do not. Mail servers absolutely do, treat a valid, forward-confirmed PTR as mandatory for outbound mail.
My PTR is a generic ISP name. Is that a problem?
For sending mail, yes. Generic reverse names are strongly associated with spam and dynamic IPs. Set a dedicated hostname that forward-confirms.