PTR record (reverse DNS)
A PTR record maps an IP address back to a hostname, and receivers use it to check that the sending server is who it claims to be.
A normal DNS record turns a name into an IP address. A PTR record does the reverse and turns an IP address into a name. Receiving servers often look up the PTR of the connecting IP. A good setup is forward-confirmed: the IP points to a name, and that name points back to the same IP.
You do not set PTR in your own domain's DNS. The owner of the IP address sets it, which is your hosting company or mail provider. If you send through Google Workspace or Microsoft 365, their servers already have proper PTR records and you have nothing to do.
PTR matters when you run your own SMTP server or use a dedicated IP. A missing or generic PTR (such as a long hostname made from the IP) is a common reason for rejection. Gmail and Yahoo list valid forward and reverse DNS among their sender requirements. If you manage your own IP, ask the provider to set a PTR that matches your mail server name.