Lost as in they are neither in the local archive folder nor inbox? Is the accout POP or IMAP?

IMAP account. A group of emails was selected in the Inbox. The archive function is selected in the menu. The backup was completed without errors. After archiving, we found a problem with emails in the folder that the program created itself during archiving.

What happens when you open (double-click) one of the messages that appear lost? Are their contents loaded as expected?

There is no information. No subject, no content, no sender and recipient data. There is nothing. Screenshot attached

Since this is an IMAP account, log in to your email service provider's webmail interface and check if the archive folders and their contents are intact. Also check if those messages are still in the Inbox.

Webmail checked. There are no email messages in the inbox. The archive was not created on the server. Mail has been archived locally. The task was to free up some of the space on the server by archiving mail in a local folder using the Thunderbird email program.

Alright. Thunderbird stored downloaded messages in mbox files inside your Tbird profile folder. It creates one mbox file for each mail folder; for example, the Inbox, Sent, Archive and Trash folders have identically named mbox files in the Tbird profile folder, and each of those mbox files stores ALL the messages that the corresponding mail folders contain. You can read more about Thunderbird's profiles here Profiles - Where Thunderbird stores your messages and other user data

As per that article, find your profile folder and copy the mbox files to a convenient backup location. The mbox files don't have an extension, and they will be inside the ImapMail\<server name> subfolder. They are big, depending on how many messages they contain. The inbox mbox file will be named "mbox" and it should be quite huge. Those files are in use by Thunderbird when it's running, so it is recommended that you first quit/exit thunderbird, then copy them to a backup location. You can then open the inbox mbox file in your profile folder using a text editor like notepad and inspect its contents to confirm that it still has your messages. You can use the search term "From - " (without quotes) to find each message inside the mbox file. You can do the same to inspect the contents of the archive mbox file(s).

While inspecting the inbox mbox file, search for the lines starting with "x-mozilla-status: " without quotes, and note the codes they have after the full colon. If the code is not 0001, then replace it with 0001 and save the changes while Tbird is NOT running. Relaunch Thunderbird and the messages should be back in your inbox.

The inbox contains only information about new emails. Part of the archive file with corrupted mail messages looks like this: From - Tue Jul 07 08:19:47 2020 X-Mozilla-Status: 0001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Tue Jul 07 08:19:47 2020 X-Mozilla-Status: 0001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Tue Jul 07 08:19:47 2020 X-Mozilla-Status: 0001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Tue Jul 07 08:19:47 2020 X-Mozilla-Status: 0001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Tue Jul 07 08:19:47 2020 X-Mozilla-Status: 0001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Tue Jul 07 08:19:47 2020 X-Mozilla-Status: 0001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Tue Jul 07 08:19:47 2020 X-Mozilla-Status: 0001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Tue Jul 07 08:19:47 2020 X-Mozilla-Status: 0001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

From - Tue Jul 07 08:19:58 2020 X-Mozilla-Status: 0001 X-Mozilla-Status2: 00000000 X-Mozilla-Keys:

re :The backup was completed without errors. .......I'm not sure you mean you created a backup.

Do you have a backup copy of your profile name folder ?

Do you have an antivirus product installed that is allowed to scan any folder? What AV do you use? I would recommend you do not allow AV to scan any thunderbird files and folders.

When you move an email out of an imap folder into eg: Local Folders mail account, you are deleting it from the server. It is not a simply case of copying from one file to another on computer. There is communication with the server for each email and if you have something scanning opened files and emails that are moved. I'm wondering if the AV has interrupted the process.

If I need to get a bunch of emails off server to create space in server, I usually synchronise all folders for offline use to ensure I have a full up to date copy and then in Offline mode ( If you cannot read emails in offline mode then a full copy has not been downloaded) 'copy' emails to Local Folders. If the 'copy' fails then I can repeat as original is still in same place. Once I have good copies, then I delete emails from Imap folder and go back to online mode, so allowing imap folder to synchronise with server.

I have installed ESET Endpoint Antivirus 7.0.2091. There were no visible errors when copying. Copying was without an offline copy. Is it possible to recover data or not?

Is it possible to recover data or not?

Not possible without an intact copy of the mbox files.

Thank you for your help!

Suggest you do not allow scanning of Thunderbird profile folders and files.

• https://help.eset.com/ees/7/en-US/idh_exclude.html

If you do not want to disable scanning of emails, then I would advise you do not allow ESET or any Anti-Virus to auto fix because it is likely to either corrupt the Inbox or delete it all together, so you end up only seeing new mail downloaded thereafter.

Emails are important documents and therefore it is advised a copy/backup is created every so often. You cannot rely on having one copy on the server (your situation) or one copy on computer.

Good info here: https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data

Have you ever copied the profile name folder ?