What it is
Normal DNS turns a name into an address. Reverse DNS does the opposite: a PTR record under the special in-addr.arpa (or ip6.arpa) tree maps an IP address back to a hostname. For 203.0.113.7 the lookup is for 7.113.0.203.in-addr.arpa, and the answer is something like mail.example.com.
The record is controlled by whoever owns the IP block — your hosting provider or ISP — not by your domain's DNS. That is why it is set in the VPS control panel rather than alongside your MX records.
Forward-confirmed reverse DNS
Receivers do not stop at the PTR. They take the hostname it returns, resolve it forward with an A or AAAA lookup, and check that the answer includes the original IP. Both directions have to agree. A PTR that points to a name which resolves somewhere else is treated as no PTR at all.
Gmail states this explicitly: sending IPs must have a PTR record, and the forward lookup of that hostname must return the same IP. Fail it and the response is 550 5.7.25.
Why it matters for cold email
On Google Workspace and Microsoft 365 you never see this; the providers own the IPs and publish correct records. The moment a fleet moves to a dedicated SMTP relay, a VPS, or a cheap SMTP reseller, it becomes your problem, and it is invisible until the Gmail bounces start. Any setup that sends from an IP you control needs the PTR checked on day one, and ideally the hostname should be under the sending domain so that everything a receiver sees tells the same story.
Check yours
These run free in your browser. Nothing you type reaches a server.
Common questions
Who sets the PTR record?
The owner of the IP address block — your hosting provider, VPS host or ISP. It is usually a field in their control panel, sometimes a support ticket. It cannot be set in your domain's DNS zone.
Does the PTR hostname have to match my domain?
Not strictly; it has to resolve back to the IP. In practice a hostname under your sending domain is better because it aligns with everything else a receiver sees.
Do I need PTR on Google Workspace?
No. Google publishes reverse DNS for its own sending infrastructure. The requirement bites when you send from an IP you control.
Related concepts
If this is the thing going wrong
The pages explain it. If you would rather it was simply fixed, that is the work I do.