SpamAssassin Rule: SPF_NEUTRAL
Standard description: SPF: sender does not match SPF record (neutral)
SPF (Sender Policy Framework) is an open standard specifying a technical method to prevent sender address forgery. The sender's domain is matched against a list of allowed mail relays for that domain. This states, for example, that mail from email@example.com should have come via mail.example.com and not mail.badguys.info.
This often breaks where users have forwarded their email to another domain, but the forwarding mechanism is not SPF-aware. Such a user would see SPF_FAIL tags on some of their incoming mail.
The domain owner has explicitly stated that he cannot or does not want to assert whether or not the IP address is authorized. A "Neutral" result MUST be treated exactly like the "None" result; the distinction exists only for informational purposes. Treating "Neutral" more harshly than "None" would discourage domain owners from testing the use of SPF records (see Section 9.1).
From RFC 4408
The default scores for this rule can be found in the online list of tests.