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What is Apache SpamAssassin?
A widely used open-source spam filter that scores email against hundreds of rules to decide whether it is spam.
Apache SpamAssassin is an open-source mail filter that evaluates each message against a large rule set (content patterns, header checks, Bayesian learning, DNS blocklists, and authentication results) and sums the weighted hits into a spam score, tagging messages at or above a threshold of 5.0 by default. It is deployed on many corporate and ISP mail servers and is the engine behind many spam-test tools. Its checks include SPF, DKIM, and DMARC results and RBL lookups in addition to body content.
Why it matters for cold email
Because SpamAssassin, or rules modeled on it, guards a large share of the business mailboxes cold email targets, understanding its scoring tells you which content and authentication choices add penalty points. Testing copy against it surfaces fixable triggers, but remember that big-provider filters like Gmail rely far more on reputation and engagement than on these static rules.
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Stop guessing at deliverability.
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