If PTR mechanisms are detected, the current diagnostic output is:
Warning: PTR mechanisms SHOULD NOT be used and cannot be resolved using this diagnostic tool.
What does the PTR mechanism mean? When an email receiver gets a piece of email and the PTR mechanism is 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.
3 important things about the above:
- The PTR mechanism has been deprecated. See the relevant RFC for more info.
- The SPF Surveyor cannot resolve PTR mechanisms because a real connection from a real sender is necessary to complete the lookup.
- MOST IMPORTANTLY: Some large receivers will skip the mechanism – or worse they’ll skip the entire SPF record – because such mechanisms cannot be easily cached. Imagine a large receiver doing a PTR lookup for millions of different connections… the size of the local cache explodes.
