Perhaps refer to the support documentation on the subject https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages You can use the [[Config Editor]] to change the preference mail.adaptivefilters.junk_threshold to a number less that it's default 90 to make the filter less strict or increase it a bit to make it more strict. The 90 is like the algorithm is 90% sure the message is spam so increasing the value to 95 means the algorithm is more sure than it used to be before it marks a message as spam. I would suggest you actually disable the junk filtering in Thunderbird for a short time and see if anything changes. What I am seeing is a lot of folk here with spam filtering issues are complaining about filtering being done by their mail provider. So turning off Thunderbird filtering allows you to see the source of those mails you want to see being classified as spam.