Imap mail account store all emails on server. Imap folders synchronise with server to display what is on server. So in effect you have a virtual view of the server.

Even if full copies are downloaded to an imap folder, you cannot say they are a backup copy because the folder constantly is sychronising to display what is on server.

The good thing about downloading full copies means you can read emails even in offline mode and if you exit Thunderbird, you can create a backup copy of the Thunderbird profile containing those copies to an external drive, so they could be imported back into your 'Local Folders' mail account if required and then copied back onto the server.

If you have not compacted any folders, you could try this addon extension, but I'm not sure if it works with version 60* https://freeshell.de//~kaosmos/index-en.html#recDelMsg

Having everything stored on the server is still only stored in one place. Have you ever performed your own backup of you Thunderbird profile folder?

What server / ISP are you using?

Thanks Toad-Hall. Appreciate your feedback/info. I've never done any of the things you suggest re backups or full copies. I'll see if that addon extension works on my version of TB. I'm using a smallish isp, chillidog hosting.

Just wanted to say that in all of this I have had full support from chillidog hosting. I'm not suggesting the problem lies with them - it is clearly a user issue which I haven't tracked down yet. The system is too old for Toad-Hall's suggested addon but I appreciate his observations and recommendations.

What version of Thunderbird are you using? Current version is 60.6.1

Do you have an anti-virus product scanning Thunderbird folders on startup or scanning emails being downloaded ?

check anti-virus to see if it quarantined/fixed the Inbox file in your profile folders. Suggest you make thunderbird folders exempt from scanning or allow scanning so you get informed but do not allow AV to fix anything.

Unfortunately the age of the computer only allows v 45. The computer concerned is a remote one, so I'll follow up on your suggestions asap. Thanks again for your very helpful input Toad-Hall

from the Thunderbird help menu, select troubleshooting information. Use the show profile button to open the current profile on disk. CLose Thunderbird. open the imap mail folder in "I assume" windows file manager Open the folder names after the mail server. Is there a file labeled inbox and an inbox.msf.

Make a copy of inbox. open it in a text editor. Notepad will do if the file is small enough, other wise Notepad++ which you can download will do the job for you. Note there is no file extension so actually open the files using the open within the text editor. Double clicking them may well start a circular process that ends badly.

In the text editor, can you see anything that looks like the missing mail, with the knowledge that the oldest mail in the file is the first one listed. If you can see what looks like dates that correspond to the missing mail, there might be some recovery, otherwise they are a loss.

I have also uploaded a modified recover deleted message that should install on TB 45. https://drive.google.com/file/d/1yoggF2FO5N2XGKp9yeDSPFgVEil-2FD5/view?usp=sharing Not that I am any good with Google drive.

Thank you Matt. What an amazing resource this forum is. I appreciate your efforts to solve the problem. It will be another day or two before I can run you solution on the target computer. All the best

Sorry Matt but still can't get access to the remote computer to test your solution. Expect to be able to some time next week.