PTR Record

A PTR record maps an IP address back to a hostname. An A record answers which IP a name lives at; the PTR answers the reverse, what hostname owns this IP. A matching A record and PTR together form reverse DNS, or rDNS.

What reverse DNS is for

Its biggest job is outbound email. A receiving server runs a PTR check on incoming mail: it compares the hostname your server announced during the connection, the HELO or EHLO name, against a reverse lookup of the sending IP. Major providers junk or reject mail from IPs without a proper PTR because the pattern looks like a throwaway server. Without PTR, mail can be refused with an error like 550 Service unavailable, sent to spam, or quietly lose IP reputation over time. Outside email, PTR makes server logs and firewall rules readable, since one hostname tells you more than a bare line of numbers.

Where a PTR is set

PTR lives in a special in-addr.arpa zone for IPv4, managed by whoever holds the IP block, which is your hosting provider or data center, not your domain registrar. So a PTR never appears in your domain's DNS zone. On an unmanaged VPS, some panels give a self-service PTR editor while others require a support ticket. If your plan uses a dedicated IP, the PTR usually follows that IP. The same idea applies to IPv6 through a different reverse-zone format.

How to check your PTR

Run dig -x 203.0.113.10 or host -t PTR 203.0.113.10 in a terminal, then compare the result with the hostname your SMTP server announces when it sends. The two must return exactly the same name. If you send as mail.yourdomain.com, the PTR must point at that same value, not at the bare yourdomain.com.

What people miss

A PTR that does not match the HELO name is the most common error, and some receivers treat that mismatch as worse than having no PTR at all. Reputation does not move with you, so a new IP needs its own PTR. And PTR is not the only inbox factor: SPF, DKIM, and DMARC still apply, and an IP already on a blacklist is not rescued by a fresh PTR. One more trap: a naive PTR check only confirms that a PTR exists, while receiving servers actually score whether the PTR matches the HELO name.

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