A PTR (pointer) record does the opposite of the usual DNS lookup. Instead of turning a host name into an IP address, it turns an IP address back into a host name. This is called reverse DNS. For example, the IP address of a sending server might have a PTR record that says it belongs to mail12.sender.example.com.
Mail servers check reverse DNS as a basic sign of a legitimate sender. Many spam sources run on compromised home computers or temporary servers whose IPs have no meaningful PTR record, or a generic one. Google requires all senders to Gmail to have valid forward and reverse DNS for their sending IPs, and many other providers treat missing reverse DNS as a strong negative signal or reject the connection outright.
The ideal setup is forward-confirmed reverse DNS: the IP’s PTR record points to a host name, and that host name’s A record points back to the same IP. Generic names that look like they belong to a home broadband pool, or names that don’t resolve back, are treated with suspicion.
PTR records are set by whoever controls the IP address, not by the domain owner. If you send through an email platform’s shared IPs, the platform handles them. If you run your own mail server or have a dedicated IP from a cloud or hosting provider, you usually set reverse DNS in that provider’s control panel or ask their support to do it.
A Chennai logistics firm sets up its own mail server on a cloud VPS to send shipment updates. Gmail rejects some messages, citing missing reverse DNS. In the cloud provider’s panel it sets the IP’s PTR record to mail.firmdomain.in and adds a matching A record, and the rejections stop.