re :inbox repair (or erase .msf index file) deletes all the blank emails The 'Repair' itself has not deleted anything. Those emails were already previously deleted from the Inbox mbox file. The index is out of sync with what is really in the Inbox mbox file, When you 'Repair Folder', it looks at mbox file and rebuilds the index to reflect what is in mbox.

This sounds like you have something scanning Thunderbird files and folders. When the mbox file has been effected but the indexing file is still intact, it is usually an AV product trying to 'fix' a file and making a hash of it; AV has no idea about the indexing file, hence why it is usualy left intact.

It is very odd that the same identical issue is occurring symultaneously on different computers, which instantly makes you think of Anti-Virus software as it will also be on all computers.

POP mail accounts use the 'popstate.dat' file to know what has previously been downloaded so that it does not download again. Hence why, deleting the 'popstate.dat' file will download everything currently on server. However, you may get the same results if the AV is scanning and removing emails.

For one user on one computer - perform a test. Reboot computer in 'Safe Mode with networking' and then start Thunderbird. Move all good emails out of Inbox. Access profile name folder. Exit Thunderbird. Delete the popstate.dat Start Thunderbird and download emails.

Do all emails get downloaded? Look specifically for emails which were known as 'blank' before.

Note: after deleting several emails, please remember to compact the Inbox to remove all traces of old emails and free up space. This also helps to stop file corruption.

I suggest you do something about whatever AV product is scanning all your computers and producing same result on all computers. Suggest you make Thunderbird files and folders exempt from scanning. The AV could be scanning files and folders on start up or whilst downloading emails.

Please read this info on pros and cons to do with scanning. http://kb.mozillazine.org/Email_scanning_-_pros_and_cons

Remember, Thunderbird by default automatically blocks 'Remote Content'. This is something that would need to be manually done by the user.

• https://support.mozilla.org/en-US/kb/remote-content-in-messages

Recommend 'View' > DO NOT SELECT 'Display attachments inline'. This should be switched on and then off after use and only used if absolutely certain of sender and content.

If this really is the AV at work, then it is likely to cause same issues no matter what email client you choose to use if you allow AV access to files containing emails.

Reinstalling Thunderbird only fixes a few issues that specifically relate to the program itself. Most issues are to do with the profile data and not the program itself. Some issues can be made worse by a lack of good housekeeping eg: compacting folders is something that should be done on a regular basis. Inbox should be kept as an inbox for new mail and not as a general storage unit for older emails. Important Copies can always be stored in 'Local Folders' mail account, but I expect you have something performing regular backups of profile name folders.

Programs that can clean up files such as CCLeaner should not have access to TB files and folders as they have a habit of removing session and pasword files etc.

AV products should be used carefully, they should never be allowed to automatically fix any Thunderbird file or folder.