Reverse DNS works the other way round from normal DNS: it turns an IP address into a hostname. The record that does this is a PTR record, and it is set by whoever owns the IP address, not by the domain owner.
Why receivers check it
Mail servers look up the PTR record of the server that connects to them, then check that the hostname points back to the same IP (forward-confirmed reverse DNS). A server with no PTR record, or a generic one such as a home internet address, is treated as a likely spam source.
Do you need to set it?
- Google Workspace or Microsoft 365 mailboxes: no. The provider owns the sending IPs and keeps their PTR records correct.
- Your own mail server or a dedicated IP: yes. Ask your host or IP provider to set a PTR record matching your server's hostname.
This is one reason cold email from real Google Workspace mailboxes is simpler to run than from your own server.