There are questions within answers within riddles when it comes to junk processing. It is as clear as mud. I will try without promising anything. The first issue ios not all junk/spam processing is done by Thunderbird. If you use a large provider like Yahoo or Gmail or even Outlook you will be subject to provider spam filtering. Because IMAP mail is syncronised some of the stuff in your junk folder will probably have been put there by your provider. In the case of Gmail, I have observed they do not flag such mail as SPAM with the SPAM flag, so mails in the SPAM folder show with a grey SPAM flag if GMail put it there. If that is not complicated enough some antivirus products play in that space as well. Usually with SPAM and SCAM setting in the antivirus program and code they inject into the running Thunderbird like the malware they say they are trying to prevent. Like most commercial software what they do and how they do it is something of a black hole. Then you finally get to Thunderbird junk tools. Thunderbird's filtering uses your address book as a white list. (Which address books are used can be fine tuned in account settings) If a persons address is in your whitelist mail from them is never considered junk. One of the reasons I do not routinely use a yahoo mail account is they also use the address book as a white list, but mine is empty on the Yahoo web site, so their spam filter just insists on placing things in SPAM that do not belong there and I have no intention of having an extra address book online just to fix their spam filtering. Ok, that is the short version. Did it help at all?