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FastMail has for many years added an "X-Spam-hits" header to show which SpamAssassin rules were triggered by an email. Unfortunately previously finding the scores of each of those hits involved looking up a table at the spamassassin website. Now those scores have been added directly to the X-Spam-hits header immediately after each hit. So a header like this:
X-Spam-hits: BAYES_99 3.5, EXTRA_MPART_TYPE 1.091, HTML_MESSAGE 0.001, SPAMMY_XMAILER 1
Shows that BAYES_99 had a score of 3.5, EXTRA_MPART_TYPE a score of 1.091, etc. Adding these all up gives the final score of 5.5 (always rounded to 1 decimal place).