Sender Policy Framework

What is "best-guess"?

Best-guess processing is a crude, non-standard attempt at guessing the IP address range of a domain's outgoing mailservers.  "Non-standard" means it is not standardized and specific to the implementation.

The first SPF implementation that offered best-guess processing was Mail::SPF::Query.  M:S:Q's best-guess mechanism (if enabled) works like this: if a domain has no SPF record, the following synthetic record is used:

v=spf1 a/24 mx/24 ptr ?all

This is based on the assumption that legitimate outgoing mail servers are in the same class C netblock as the domain's host (a) and MXes (mx). "?" causes a "Neutral" result as a fallback, which must be treated by receivers as if no SPF check had been performed.

Other implementations may implement best-guess differently or not at all.

The practice is deprecated and should generally be avoided. While it may be useful in certain specific circumstances it's not part of the SPF protocol and results of guessing SPF like records should not be referred to as SPF results.