Spam Scoring Headers — How Filters Make Their Decisions

When a filter sends an email to spam, it doesn’t do it silently — it leaves the reasoning right there in the headers. Most senders never look. Chapter 10 is about actually reading what the filter left behind.

SpamAssassin writes its verdict into X-Spam-Status along with a full rule-by-rule point breakdown. A score of 5.5 isn’t a mystery once you see that BAYES_99 contributed 3.5 points and a formatting rule added another 1.1.

Microsoft 365 does something structurally different with its SCL, BCL, and PCL headers — three separate 0-to-9 scores for spam, bulk-mail, and phishing confidence, each with its own admin-configured threshold. A message can have a low SCL but a high BCL, landing in Junk not because it looks malicious but because it looks like graymail nobody asked for.

It also covers gateway-specific variants like Barracuda’s tag/quarantine/kill threshold system, and Microsoft SNDS as the reputation-monitoring counterpart to all of this.