When an email receiver gets an email and the PTR mechanism is integrated in the sender’s SPF record, the receiver will look at the incoming IP address and do a “PTR” lookup. For example, if the sender is sending email from IP address 1.2.3.4, the receiver will perform a PTR lookup of 1.2.3.4 to attempt to retrieve a hostname. Lastly, if a hostname is discovered for IP address 1.2.3.4, then that hostname’s domain is compared to the domain that was originally used to lookup the SPF record.