Are you talking of messages in your trash/junk folder that are not flagged as junk. That is generally mail your provider considers junk places in the relevant folder but Thunderbird does not see it as junk.

Thanks for jumping in to help out.

No, I'm talking about messages in my inbox and tagged with the tag 'NonJunk'. See image attached.

I have no idea what that is. I would assume it is something your mail provider is doing, but I must also assume they are a fairly minor player or I would think I would have heard of it here at some time in the past. But it is a new one on me.

I've got this behaviour from two different providers, one of them being gmail!

All the emails that I get from no-reply@support.mozilla.org through gmail, for example, are tagged as NonJunk.

Currently I have set up a message filter with the FiltaQuilla addon to untag messages. Not the best solution.

I do have a filter rule that marks "emails from a CardBook contact as non junk". That got there automatically somehow - I did not create that rule. Am I assuming that marking things as non junk is quite different to tagging a message as such? There is nothing in that rule that mentions tagging.

Strange. I'll keep digging and will post again if I find anything.