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The three values that can be displayed are "no" (autolearning did not occur), "ham" (the message was learned as ham), and "spam" (the message was learned as spam).

If a message has already been learned by SpamAssassin, then that message will not be learned again. Therefore, if you run a message through SpamAssassin to see why it was classified as spam or ham, and it has already been learned, you will always get the result "autolearn=no". (To see this more clearly, use the "-D" flag, and you will see debug output explaining that the message has already been learned.)

Furthermore, the score used to trigger autolearning is somewhat different than the one reported in the final score; therefore a score displayed in the headers that obstensibly should trigger autolearning will not do so. Again, use the "-D" flag to SpamAssassin, and you will see the score that is used to determine whether or not autolearning will be triggered.

For more information, please read the Mail::SpamAssassin::Conf documentation.