PTR records are the opposite of A records; rather than pointing the name to the IP, they point the IP to the domain or hostname. If you specify the PTR mechanism in your SPF data, the IP of the sending mailserver is checked through the PTR records to find its associated hostname or hostnames. The hostname(s) are then validated through A records, at least one of which must point to the IP of the sending mailserver. If a valid hostname ends in the domain name the email claims to come from, the mechanism matches. Adding this to the example we've been working with would result in the following SPF data.

You can also specify another domain which the hosts must match, through the usual syntax of ptr:otherdomain.com. The effect of this mechanism is that any server sending email whose hostname ends in the domain name specified will match, so long as its PTR and A records are set up properly.