SpamAssassin is the scoring engine most receivers apply as one factor in the spam decision. Postmark, SparkPost, Sendgrid’s mail-tester integration, cPanel/WHM, most self-hosted MTAs, and every hosted anti-spam gateway that predates the machine-learning-only era all use it in slightly different shapes. A score under 3 is clean, 3 to 5 is borderline, 5 and up will usually land in a spam folder or get quarantined at the edge. What matters more than the number is which rules fired. Each rule names a fixable defect in your headers, your MIME shape, or your authentication.
This tool sends the message you paste to 
Postmark’s free public spamcheck endpoint, runs SpamAssassin on it, and returns the score plus every rule that scored the message. Then it translates each rule name into WP-shop language and points at the relevant nanoPost fix guide. SPF-family rules go to the DNS auth checker, DKIM-family rules to the DKIM inventory, missing-headers and MIME rules to the WordPress email troubleshooting guide, HTML-weight rules to the Gmail clipping analyzer, and blocklist rules to the sending-services roundup (the almost-always-right fix for a blocklist hit is to move off the shared IP that got you listed).