What PTR record means
Reverse DNS is an ordinary lookup in a special zone. The address 192.0.2.10 becomes 10.2.0.192.in-addr.arpa, and the PTR record published there returns a hostname; IPv6 uses nibble-reversed names under ip6.arpa. Those zones are delegated to whoever holds the address block, which means the PTR for a sending server is set by the hosting provider or the ISP — not by the owner of the domain in the mail.
Receivers check it two ways. Many require only that one exists: RFC 1912 has said since 1996 that every internet-reachable host should have a matching name, and a connection from an address with no PTR, or with a generic one that spells the address out in dotted form, is a common reason for refusal at the connection stage. Stricter receivers require forward-confirmed reverse DNS, where the PTR returns a name, that name's A record returns the original address, and the two agree.
The name should also match what the server announces in HELO or EHLO, and should ideally be a hostname in the sending domain rather than the provider's default. Changing it is a request to whoever runs the address block; no record in your own zone can do it for you.
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