Quick answer
A PTR record maps an IP address back to a hostname, which is the reverse of what an A or AAAA record does. It is the record type behind reverse DNS (RFC 1035 §3.3.12). The record does not live in your own domain's zone: whoever controls the IP address block publishes it, usually your hosting provider, cloud platform, or ISP. You can look up the PTR for any address with the reverse-DNS lookup tool.
What a PTR record actually is
A forward lookup answers "what is the IP for mail.example.com?" and returns an A record (IPv4) or AAAA record (IPv6). A reverse lookup asks the opposite question: "what hostname belongs to 203.0.113.10?" The answer comes from a PTR record.
The RDATA of a PTR record is a single domain name (RFC 1035 §3.3.12). That is the whole payload. The name 10.113.0.203.in-addr.arpa might hold a PTR pointing to mail.example.com, and nothing else. There is no rule that a PTR match any forward record, and DNS itself does not enforce that the two agree. Whether they agree is exactly what mail servers check, covered further down.
Where a PTR record lives: in-addr.arpa and ip6.arpa
Reverse DNS has its own branch of the namespace. IPv4 reverse records live under in-addr.arpa, with the four octets of the address written in reverse order (RFC 1035 §3.5). The reversal is what lets a provider delegate one network's worth of reverse space as a single zone.
IP address: 203.0.113.10
PTR query: 10.113.0.203.in-addr.arpa
Read the octets right to left. 203.0.113.10 becomes 10.113.0.203, then .in-addr.arpa is appended.
IPv6 uses ip6.arpa instead and goes finer-grained. The address is expanded to its full 32 hex digits, each nibble becomes one label, and the nibbles are written low-order first (RFC 3596 §2.5). The RFC's own example maps 4321:0:1:2:3:4:567:89ab to b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.0.0.0.0.1.2.3.4.ip6.arpa. The principle matches IPv4: reverse the address, append the special suffix.
Who publishes a PTR record
This is the part that trips people up. A PTR record is not in your domain's zone file, so you cannot add it in the same DNS panel where you manage your A, MX, and TXT records. It lives in the reverse zone for the IP block, and that zone belongs to whoever owns the address space.
For a server you rent, that owner is your hosting provider, cloud platform, or ISP. You set the PTR through their control panel, where many clouds expose a "reverse DNS" or "PTR" field on the instance or IP object, or by opening a support request. If you run your own address block, you can publish PTR records in the reverse zone directly, and larger providers sometimes delegate that zone to you with NS records so you manage it yourself.
One PTR per IP is the normal case. Publishing several PTR records for a single address is technically legal but discouraged, because consumers generally expect one name back.
Forward-confirmed reverse DNS (FCrDNS)
A PTR on its own proves little, because the IP owner can point it at any hostname they like. Forward-confirmed reverse DNS closes that gap. A PTR is forward-confirmed when the hostname it returns also resolves, through an A or AAAA record, back to the original IP address.
The check is a round trip:
203.0.113.10 --PTR--> mail.example.com
mail.example.com --A--> 203.0.113.10 (match: FCrDNS passes)
If the forward record points somewhere else, or does not exist, FCrDNS fails even though a PTR is present. Mail receivers run this check on the connecting IP as a cheap, hard-to-forge signal that the sender controls both the address and the name. RFC 8499 §7 defines the reverse DNS terminology this builds on.
PTR records and email deliverability
For a mail server the PTR is close to mandatory. A missing PTR, or a generic one that still carries the provider's default naming such as 203-0-113-10.static.example-isp.net, reliably pushes outbound mail toward spam folders and rejections. Large receivers read the absence of a proper reverse record as a low-reputation signal. For IPv6 senders the bar is higher: major providers effectively require a valid PTR before they will accept the connection at all.
This page defines the record type; the mail-specific errors have their own guides. If Gmail rejects your mail with a reverse-DNS error, see the Gmail 5.7.25 PTR error page. If your PTR hostname does not match what your server announces in HELO/EHLO, see reverse-DNS HELO mismatch. To see what resolves for any address right now, run the reverse-DNS lookup tool. For the forward side of the same IPv6 address, see the AAAA record guide, and for the wider family of records, the DNS record types hub.