From what I read here. https://bugzilla.mozilla.org/show_bug.cgi?id=1648706 and looking at the source code referenced. I am guessing that the filter is created if you set the return receipt handling in preferences to move it to my send folder.

Sorry, didnt solve the issue...

included a picture of the problem filter...

it still is an empty filter but it is causing problems...

After a lot of research I found out that The problem started with TB3.

You were right about the return receipt handling in preferences but on the other hand 'if I cant choose between the INBOX or the SENT box without causing problems then dont give me a choice and send it by default to the INBOX!'

Don't get me wrong, I love Tbird but this bug is a real pain and has been there for too long.

Thank you for your answer even if the problem is not really resolved.

Chris

I have this problem Too. How do I delete this so that the problem goes away?

in Accounts,preferences the return receipt handling do not send it to the SENT box, that's what causing the problem. Leave the return receipt handling in the INBOX. Worked for me but like i mentionned above it doesnt solve the issue...

I thank you for the response. I will try this. I changed global and I change local. I hope this works. Thank you for your response.

Glad to be of help!