Thunderbird moves messages back into junk after you mark them as "not junk."

Let me add that this is a personal E-mail account at my own domain, at a traditional hosting provider. There are no automated spam filters running on the server that would move the messages.

1. What is the hosting provider?
2. Do the messages in your spam folder have a red flame icon attached to them in the message list
3. Most importantly what antivirus product do you use.

SPAM message processioning is relatively simple in Thunderbird and adheres to the information in the article you linked. That your experience is different is also not something I see as a suprise. Despite assertions to the contrary rarely do hosting providers not offer, and usually enforce, some sort of SPAM monitoring or exclusion. Your description of mail being returned to the spam folder after you make a manual decision is reminiscent to what happens at Yahoo. They constantly monitor all folders and just put the mail back in SPAM until such time as you buckle and upload your address book to their web site so their spam system can whitelist your contacts. An appalling situation, but most Yahoo customers just accept it.

Then we have the junk toolbars from a plethora of antivirus vendors that tell you they are protecting your from scams. but meddle well outside that area because they can. In the long term it usually makes Thunderbird unstable, but the third party antivirus product is "bullet proof". Interestingly Microsoft Defender scores well in virus detection/malware detection tests and does nothing with email at all. It is also not selling a subscription.

My guess if the change is happening in seconds as you say is local software. A mail server moving mail twice would not normally happen in seconds. A minute maybe but unlikely to be seconds. So I would start by pursuing any toolbars or offers to monitor scam in your antivirus product of choice.

Thanks for the reply.

The host is Nixihost. They use SpamAssassin but it only puts "SPAM" in the subject line of messages deemed suspicious. Similar messages I receive on my home system (a Mac, using the native Mail app) are not bounced into Spam.

I do not have any junk toolbars. This is a work environment, and the only antivirus on the system is whatever Fortinet provides. All company-related E-mail goes through Outlook. I installed Thunderbird long after the system was set up, and use it only for my personal account. So I would be surprised if there were some integration between Thunderbird and an antivirus.

The problematic messages do not have a flame icon; at least this was true for the last one I observed. Other automatically-moved spam messages lack the flame icon also, but many do have it.