Spam score

What Spam score means

Rule-based filters — SpamAssassin is the one whose output most people see — test a message against hundreds of rules and sum the weight of each that fires. Some rules look at content, some at the headers, some at the sending infrastructure: a missing Message-ID, an HTML part with no plain-text alternative, a body that is one large image, a link whose host is on a URI blocklist, an SPF failure. The default threshold is 5.0, and the report names every rule that contributed to the total.

A spam score is one filter's opinion, not a verdict on deliverability. Large mailbox providers do not publish their scoring, and they weigh sending reputation — how this domain and this address block have behaved — alongside content in ways no public scorer reproduces. A message can score cleanly and still land in a spam folder because the address sending it is new.

That still makes the score worth reading, because the rules it names are specific and fixable. Authentication failures, a missing List-Unsubscribe on bulk mail, a Date header that disagrees with the trace fields and text that trips well-known phrase rules are all worth correcting before looking anywhere else.