Networking
Reverse DNS
PTR on the server IP — needed for mail, useful for identification.
Where to set it
rDNS is set from the server page when that control is on. It is not a DNS record you add in the domain editor: it lives on the IP, not in the domain zone. The hostname you set should match a forward A record (the name resolves to that IP).
Outbound mail from a VPS is often rejected if PTR is missing or does not match HELO. Set rDNS to the name your mail server uses, then confirm the A record for that name returns the same IP.