Then turn off junk filtering. It is very very simple and you appear to have already done so. See https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages

However there are lots of posts to this forum from folk that are unaware their providers junk filtering applies to IMAP mail accounts, as does whatever their antivirus product comes with. See https://support.mozilla.org/en-US/questions/1531532 as an example.

Historically a sudden huge change in behavior of long standing accounts are generally issues with some sort of spam/scam filtering imposed from an antivirus product. Kaspersky has been getting lots of recent messages. That does not however exclude the provider inventing a new spam policy. Yahoo for instance ignores anything done in the mail client and continuously monitors all folders in the account and moves mail back to their bulk mail folder regardless of the users choices. You have to add the sender to their website contact to get any form of them not moving mail for that person.

First, thank you, Matt, for your fast and helpful response to my recent inquiry. My husband and I appreciate your taking the time to respond with suggestions to solve the problem I described.

We checked the Spam and Junk criteria from our email provider (Charter/Spectrum) and found little evidence of filtering that was occurring. Still, we found a place to tell Charter to mark any incoming message it “thinks” are Spam with that designation and to place them in the Inbox. That was successful, but only for a very few incoming emails, perhaps 4 or 5 messages, since we made that change. We still had many, many messages going to the Junk folder.

Next, we tried deleting the Thunderbird program altogether and reinstalling it. We then turned off all Junk filtering and set it to put everything into the Inbox and nothing into Junk. At first, it seemed to be working. We went for several hours with no new messages showing up in the Junk folder. Then, this morning, Thunderbird was back to its newly familiar routine of throwing lots of messages into the Junk folder. I just now checked again and noticed an email just now placed in Junk that had only just been sent from a Bonafide company sending me a special coupon to make up for a problem the company had caused with an order.

In addition to the Junk issue, I also discovered one instance in which Thunderbird somehow had placed items into the Trash folder all by itself. These were messages I had not sent to Trash. Fortunately, that happened only on that one occasion -- that I know of. I don’t review the contents of Trash because I am the one who designates a message as trash. However, interestingly, I have not received the usual notice that a statement is available from several creditors in the last 2-3 weeks, so I suspect the Trash folder has been affected by whatever odd behavior Thunderbird is exhibiting.

Another recent issue involves messages I am confident exist that are still in Thunderbird but hidden from view, as in not in either the Inbox or any of my folders. In some cases, I have successfully located a message via the search function. A recent example of that issue is the message from Mozilla Support Forum with the Subject “Matt posted an answer to your question ….” Oops!

So, although I was hopeful that I could solve my Thunderbird problem, in the end, I don’t believe the problem -- and now problems -- with Thunderbird has/have been resolved. Further, unfortunately, I don’t think they are fixable. It is such a significant issue that my husband and I are now exploring other email program options. We’ve really liked Thunderbird over the past several years, but it is no longer dependable and cannot be counted on to perform as expected.

Again, thank you very much for responding to my inquiry. Denise

It will be helpful to know exactly what you are seeing.

In the spam/junk folder, please add the column spam/junk to the message list. Then screen shot the list of messages seen in the folder, as seen in the screen shot I am providing.