See the 7th item under 'Common Problems' here. I suggest following the instructions given there to rebuild the Inbox, which essentially involves moving the current contents to another folder and deleting the Inbox mbox file in the profile folder.

Thank you for your help.

I tried emptying the inbox, deleting the 2 files, and restarting TB. I did the same for Sent and Trash.

I also deleted all .msf files for all inbox sub-folders and for all folders and sub-folders under "My Mail".

the problem persists.

My inbox has many sub-folders and sub-sub-folders should those be moved to My Mail?

Another problem I am having. I do not know if it is related. I have a stack of filters that move incoming mail into folders. The stack ends with 2 filters.

(1) a filter that checks whether FROM is from someone in my mail lists. It then moves the message into a folder called "Known Sender".

(2) a psuedo-ELSE filter that check whether the subject contains a very improbable string. It put the message in a folder called ") Suspicious". Sometimes I still end up with folders in Inbox that clearly should be in the two folders, "Known Sender" or ") Suspicious".

You have to move all mail from Inbox to another folder, then exit TB and delete Inbox and Inbox.msf from the profile folder.

http://kb.mozillazine.org/Compacting_folders#Real_fix

Then, any new messages in Inbox should have the correct format.

I expect that message filters will work properly once the Inbox is rebuilt.

Do you mean I should remove all sub-folders and all sub-sub-folders from inbox?

Should I move messages from Inbox folders, sub-folders and sub-sub-folders to "My Mail" or would it be better to move them to local folders?

The article suggests you only have to deal with Inbox, not Inbox.sbd (subfolders). Move them to Local Folders. It wouldn't hurt to back up the profile before making any changes.

Thank you. I'll move all my inbox folders, sub-folders, and sub-sub-folders from My Mail to Local folders.

Should I move all my other My Mail Folders to Local folders or only the ones that are sub parts of inbox?

WRT backup, I run Mozbackup and also copy the .default file to desktop before I do any of the attempts to fix these problems.

You only have to move mail that belongs to a folder that needs repair. If only Inbox is affected, but not its subfolders, then you can keep the subfolders where they are.

How do I know which folders need repair?

3 times I have: moved all messages out of the inbox itself into sub-folder, sub-sub folders, and other folers, etc. under My mail. run mozbackup copied the profile (.default) to the desktop deleted inbox and inbox.msf from the profile

the problem persists.

If you are still receiving messages with damaged headers, in a new, empty, rebuilt Inbox folder, then there must be some external factor that is corrupting the Inbox, even after it is rebuilt. The most likely factor would be an antivirus program that is acting on the mail folders.

Thank you for your frequent replies.

I only have the Microsoft Defender. I'll turn it off for a while. During that time I'll not use Firefox or anything else that connects to the internet. I'll try TB a couple of times. It is almost 5pm and a lot of mail should come in the next couple of hours.

I still received the message with 1969 dates.

I do not know if this may be causing some of the problems. MozBackup produces the message in the attached clip about unknown files. Are these files I should remove from the profile?

The only ones I can't recognize are the first, .s file, and desktop.ini, neither of which are normal profile files.

It's probably time to bite the bullet and create a new profile with Profile Manager, set up your accounts, and then transfer your mail and contacts from the old to the new profile, incrementally, so you can recognize any corrupted elements. It will be faster and more effective than trying to isolate the problem.

http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Thunderbird

To start PM, Winkey-R and type thunderbird.exe -p

I am about to create a new profile again.

Since your advice I created a new profile twice. Also all my messages that I need to keep are now in Local Folders. Messages remain i Inbox only until I delete them or move them on a local folder.

What I hope is my last point to clarify is: Can a message folder (one with no filename extension aka file type) still carry corruption? Do I need to move folders a few at a time and then wait until I use TB to see if problems remain?

If you're referring to the mbox files in the profile folder, the ones with no extension, they can be corrupted, e.g. as indicated by the 1969 dates in messages. As long as there is no process causing corruption on an ongoing basis, it would be best to bring in mail folders from old profiles one at a time to check for problems.