What Is a PTR Record?
TL;DR
A PTR record is a DNS record that maps an IP address back to a hostname, the reverse of an ordinary lookup. Mail receivers check it on the connecting address, and Google requires every sending IP to have one whose hostname resolves back to that same address.
Where is a PTR record published?
Not in your own zone. Reverse records live under a dedicated namespace, in-addr.arpa for IPv4 and ip6.arpa for IPv6, in which the address is written backwards, so the record for 203.0.113.10 sits at 10.113.0.203.in-addr.arpa.
That namespace is delegated to whoever holds the address block, which means your hosting provider, cloud vendor or ISP. You cannot add a PTR record at your registrar, and on shared sending infrastructure you often cannot set it at all.
Why do mail receivers check the PTR record?
It is the cheapest signal that an address belongs to a real mail server rather than to a compromised home connection. Google states the requirement in both directions: the sending address must have a PTR record, and the hostname in that record must have an A or AAAA record pointing back to the same address.
A missing or generic reverse name is a common reason mail from a new server is refused at connection time, before any authentication or content has been examined.
What should the hostname be?
A name in a domain you control that reads as a mail server, and that matches what the server announces in its SMTP greeting. Consistency across the three, greeting, forward record and reverse record, is what is actually being checked.
Leaving the default name a provider assigns is what causes trouble, because those names read as generic infrastructure rather than as a mail host. On a shared platform the provider owns this and there is nothing for you to set.
Frequently asked questions
No. The record belongs to the reverse zone for the IP address block, and that zone is controlled by whoever owns the addresses. The request goes to your hosting or cloud provider, not to your registrar.