Reverse DNS (PTR) for mail
A PTR record maps an IP address to a hostname in the in-addr.arpa or ip6.arpa tree. RFC 1035 defines PTR. Some receivers compare the PTR name to the EHLO hostname presented during SMTP.
The record
2.0.192.in-addr.arpa. IN PTR mail.example.com.
RFC 5321 recommends that the domain name given in the greeting resolve to the client host. Google's SMTP error list includes rows where Gmail rejects mail when forward and reverse DNS do not match.
IPv6
IPv6 uses the ip6.arpa domain. The nibble-oriented delegation follows RFC 3596. Gmail's list includes a row for IPv6 senders whose PTR records do not validate.
PTR is not SPF or DKIM. It is a DNS hygiene check some large receivers apply before or alongside authentication.
EHLO recommendation
RFC 5321 recommends that the domain name given in the greeting resolve to the client host. Operational practice calls that forward-confirmed reverse DNS.
The 550 5.7.25 row on Google's SMTP error list names PTR problems for IPv4 senders. IPv6 senders have a separate row when PTR validation fails.
Publishing PTR is done at the IP provider or regional registry delegation for the address range, not in the same zone as the domain's MX unless the operator controls both.