Mail marked as "Not Junk" is returned repeatedly from the Inbox to the Spam Folder
I always train the junk filter by marking mails that are erroneously sent to the spam folder as "Not Junk". They are transferred back to the Inbox as one would expect, but after a few minutes some (not all, but most) are again moved from the Inbox to the Spam folder. This happens repeatedly every time I mark these legitimate mails from addresses that should not arouse suspicion anyway, such as theguardian.com (an address that is in my address book, so should be whitelisted anyway) or Arri.com, just to name two current examples. Why is Thunderbird overriding my "Not Junk" flag? How can I train the junk filter if Thunderbird ignores this? Is there something in the Settings I'm missing?