Reverse DNS (PTR)
Reverse DNS maps an IP address to a hostname — the inverse of ordinary resolution. Receiving mail servers check it: an IP without a coherent PTR carries a heavy handicap from the start.
In practice
The golden rule for outbound mail: the PTR of the sending IP, the hostname the server announces, and its forward resolution must form a consistent loop. Gmail rejects IPs without a PTR — we encountered this on a first test send that went out over IPv6 without reverse DNS: immediate rejection, fixed by forcing IPv4 output with a valid PTR.