The "ptr" mechanism (edit)
The hostname or hostnames for the client IP are looked up using PTR queries. The hostnames are then validated: at least one of the A records for a PTR hostname must match the original client IP. Invalid hostnames are discarded. If a valid hostname ends in domain, this mechanism matches.
If domain is not specified, the current-domain is used.
If at all possible, you should avoid using this mechanism in your SPF record, because it will result in a larger number of expensive DNS lookups.
Examples:
"v=spf1 ptr -all"
A domain which directly controls all its machines (unlike a dialup or broadband ISP) allows all its servers to send mail. For example, hotmail.com or paypal.com might do this.
"v=spf1 ptr:otherdomain.com -all"
Any server whose hostname ends in otherdomain.com is designated.
