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SenderProbe
Reputation check

Check the PTR record behind a sending address

A receiving server asks who an address says it is before it accepts anything from it. SenderProbe resolves the addresses behind a domain, reads the PTR record for each, and confirms that the name it returns resolves back to the same address.

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What a PTR record is, and who can publish one

A PTR record maps an address back to a name. It lives in a zone derived from the address itself: under in-addr.arpa for IPv4, with the octets reversed, and under ip6.arpa for IPv6, one nibble per label and likewise reversed.

You cannot publish it in your own zone. The reverse zone is delegated with the address space, so the PTR record for a mail server is set by whoever assigned you the address — your hosting provider, your ISP, or your own organisation if it holds the allocation. This is deliberate: it means a PTR name is a claim the network operator has agreed to, not one the sender made up.

A matched pair for 203.0.113.10: the PTR names a host, and that host resolves back to the same address.
10.113.0.203.in-addr.arpa.  3600  IN  PTR  mail.example.com.
mail.example.com.           3600  IN  A    203.0.113.10

Forward-confirmed reverse DNS

A PTR record on its own proves little, because the operator of an address can point it at any name at all. Forward-confirmed reverse DNS closes that: the receiver takes the name from the PTR record, resolves it, and checks that one of the addresses it gets back is the address that connected. Only then is the name treated as belonging to the sender.

This is the check SenderProbe runs, and rDNS that is present but not forward-confirmed is reported separately from rDNS that is missing. The two need different fixes: one is a missing A record, the other a missing PTR record.

Why a generic PTR name gets mail filtered

Providers give every address a default reverse name, and those names are recognisable: they contain the address in dotted or dashed form, or words such as dynamic, pool, dsl or static. Filters read them as a host that was never intended to send mail directly, because addresses with names of that shape overwhelmingly are.

Replace it with the hostname the mail server actually uses. A PTR pointing at a name under your own domain, resolving back to the same address, is one of the cheapest reputation improvements available to a self-hosted sender.

Making the PTR, the HELO name and the A record agree

Three names should be the same: the name your mail server gives in its EHLO command, the name in its PTR record, and a name with an A record resolving to the sending address. Receivers compare them, and a mismatch is a common scoring rule in spam filters.

It is not a formal requirement of RFC 5321 — a receiver may not refuse mail solely because the EHLO name does not verify — but a permissive specification and a strict filter are different things, and the filter is the one deciding whether your message is read.

Reverse DNS for IPv6

IPv6 reverse zones are the same idea at an awkward scale: every one of the 32 nibbles becomes a label, reversed, under ip6.arpa. Nobody writes these by hand for long, and providers usually offer a form or an API instead.

Set it. Several large receivers refuse mail over IPv6 from an address with no reverse DNS at all, applying a stricter rule than they do to IPv4 precisely because an IPv6 sender is expected to have configured one deliberately.

The reverse name for 2001:db8::10, one nibble per label, in reverse order under ip6.arpa.
0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa.  IN  PTR  mail.example.com.

Run this check on your own domain. A workspace takes a minute to create and runs every one of the eleven checks against the domain you name.

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Common questions

What is a PTR record?
A PTR record maps an IP address back to a hostname. It is published in a reverse zone derived from the address, under in-addr.arpa for IPv4 and ip6.arpa for IPv6, and it is controlled by whoever holds the address allocation rather than by the owner of the domain the name belongs to.
Who sets the PTR record for my mail server?
Your hosting provider or ISP, because the reverse zone is delegated along with the address space. Most offer a control panel field or an API call for it. If you hold your own allocation from a regional registry, the reverse delegation is yours and you set the PTR record in your own reverse zone.
What does forward-confirmed reverse DNS mean?
It means the PTR record for an address gives a name, and that name resolves back to the same address. Receivers check both directions because a PTR record alone can name anything the address operator likes. A reverse DNS lookup that is not forward-confirmed is treated by many filters as though there were no reverse DNS at all.
Does missing reverse DNS stop mail being delivered?
Frequently, yes. Several large receivers refuse mail from an address with no PTR record, and over IPv6 the rule tends to be stricter still. Others accept the message but score it heavily against the sender. It is one of the few requirements that is enforced at the connection, before any content is seen.

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