This may apply to your online email account. That is, the online email account may have moved the message to trash prior to your starting Thunderbird.

Thanks David for your advice...

- I was checking all common settings related to online email account and could NOT find anything wrong ...which obviously DOES NOT MAKE ANY SENSE (!):  because the settings in the Email account seem to comply with the suggested setting, this type of issue really annoys me since it clearly indicates an error caused by poor work of the responsible Programmer ...

Anyway, I would greatly appreciate an advice of anyone who would know how to eliminate this 'mysterious' error ... Tony H.

You need to tell us more about your mysterious error.

I've used the word 'mysterious' because after about 35 years of using all previous Windows versions together with 4-6 years of using steadily (and ONLY) THUNDERBIRD Online Email Account, I've NEVER experienced this unexpected issue which has happened just a few days ago: during my daily usage of THUNDERBIRD-Email Account, I've noted one day that the INCOMING messages started to collect in the 'TRASH' folder instead of in the originally-set 'Inbox' ...and unfortunately - have no clue what has caused this problem.

One thing 'might have happened': I've checked the Properties of the TRASH folder and it is stated there that ....the Folder Index (.msf) file becomes damaged and by using the 'Repair Folder' may fix these issues ...however, I'm afraid to use it not knowing if it could complicate rather than fix this problem BUT - has anyone a 'good experience' with using this 'Repair Folder' option in the TRASH folder ...? Any further advice would be greatly appreciated ...Tony H.