Created Folder and Moved E-Mails - E-Mails Disappeared

Is this an IMAP account or POP3? If IMAP, did you create a local folder or one on the mail server?

What version of Thunderbird?

What is your operating system?

What is your anti-virus, if any?

This should be a reminder to make regular backups of your Thunderbird Profile!

Thank you for your reply. I will answer your questions:

It is an IMAP account

The version of Thunderbird is: 151.0.1 (64-bit)

I still use Windows 10 but I intend to upgrade to Windows 11 once I get this problem resolved

The antivirus is Malwarebytes

I created the folder and put precious e-mails in it and next time I opened they were no longer there. Do I need to take backups constantly?

Thanks again for your help.

Is this an IMAP server folder? Is it visible when you view your account on the provider's web mail interface?

The only times I've seen folders seem to disappear on their own is when the computer's Anti Virus thinks there is a suspicious attachment embedded in the folder file and quarantines the entire folder. Check Malwarebytes to see if that has happened.

I've also seen where people have accidentally drag-and-dropped a folder file onto another. If it's not obvious that that has happened, a way to check is to use the Global Search function to search for a known message.

Backing up your profile "constantly" would be overkill, but as with all valuable data, a backup and recovery plan is essential. All computers eventually fail if they are in operation long enough. It's just a fact of life.

Thank you for your reply.

It is an IMAP server folder. It is not visible when I view my account on the provider's web mail interface.

The folder did not disappear. It is still there. Alas it is empty as all e-mails which I put there are no more therein.

I could not see any indication in Malwarebytes that it quarantined anything. In any case some of the e-mails I have had for weeks if not months while others were new. At least all were from this year.

The fact that the folder had some older messaged in it is irrelevant. If Malwarebytes thinks there's a suspicious attachment it will quarantine the entire folder, not just a single message.

I'm a little confused. The folder itself is not visible at all on the server? But it used to be?

Did you check the Trash folder to see if the messages were deleted in a conventional manner? Obvious, I know but...

Have you used the Search featured (<Edit | Find | Search | Search Messages or Global Search> to look for a known message?

Do you have any other devices monitoring this mailbox that might be set up to delete messages?

Can you search your hard disk for a previous copy of the errant folder? Search for both the folder name and (folder name).msc.

Have you checked your hard disk for corruption?

Otherwise I'm running out of ideas. This is most unusual. I have been using Thunderbird for decades and it has happened to me.

Thanks for your reply. I will answer your questions.

The folder is not visible on the server of the provider and never was, I am sure. Why would it be?

I checked Trash and Archives folders.

I used the search feature to locate the messages. I do not know where to find Edit.

No other devices monitor this mailbox.

I have just searched the hard disk for copies of the folder. Alas I found nothing.

I have the folder but it is empty. Its e-mails are missing.

I have not checked the hard disk for corruption. I do not know how to go about that. I have not experienced any corruption otherwise.