SpamAssassin Rule: SPF_HELO_NEUTRAL
Standard description: SPF: HELO does not match SPF record (neutral)
SPF (Sender Policy Framework) is an open standard specifying a technical method to prevent sender address forgery. The domain in the HELO command 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.
In a normal mail client, the HELO command uses the internet name of the computer sending the mail, so that someone might use their computer 1-2-3-dyn.bigisp.com to send mail through bigisp.com's mail relay, which has an SPF record indicating that that's allowed.
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.