The junk feature is often assumed to be a whitelist/blacklist function, but it is not. Marking a message from a sender as junk does not place the sender into a blacklist, as Thunderbird uses an algorithm to detect junk. I suggest a thorough review of the article at https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages One of the requirements to effectively train the junk controls is to routinely mark both junk and non-junk. Some users also use third-party software, such as Mailwasher, to assist in that.