FORGED MUA OUTLOOK - Spamassassin Wiki

If you get false positives due to FORGED_MUA_OUTLOOK hits, here are some fixes (pick one) for the admin to implement to solve this problem:

upgrade to the newest version of SpamAssassin (better solution)
edit the line in the file 50_scores.cf, replacing the #s with smaller #s (e.g. 0 or 1).

This already-fixed bug happens because a new version of Outlook wasn't recognized by the rules of the old SpamAssassin installation used on your mail.