pLease post a screen shot with the contents of "Local Folders" in the "Mail" folder of your Thunderbird profile.

Thank you for your reply

Thanks for the image. I can't see any anomaly in your screen shot besides the fact that all message files are empty and haven't been actualised (used) since some years except for Trash. Maybe the message loss has happened in subfolders of Inbox, Sent or Archive. In this case I would have to look at the contents of the Inbox.sbd, Archive.sbd and Sent.sbd folders.

This is a portion of a subfolder inside the Sent.sbd folder where the main problem is... Some of the folders contain the messages normaly and some of them don't show any messages at all. mycleverbox is empty and mydreamforyou is normal for example.

Some of your *.msf index files are rather small in comparison to the size of the corresponding mailbox file. Thunderbird closed, delete the mycleverbox.msf file in the profile folder and restart TB. A new mycleverbox.msf file will be created. Verify the contents of the mycleverbox folder in TB and report whether this helped or not. Note: sorry, I forgot to add the .msf extension to mycleverbox. I repaired the error today 4/27/2025.

Hello,

I have already tried this... When i remove the .msf file, TB automatically recreates it but the file has 0 bytes and still no messages appear... Its is a very wierd behavior of TB and I do not know what caused it..

have you tried right clicking the folder, selecting properties and then using the repair button. It is essentially the same thing as deleting the MSF, but if something is preventing the proper creation of the MSF index, doing it within the application might remove that issue.

Hello,

I have already tried this approach multiple times in multiple folders but the process seems to "stuck". At the bottom bar of TB I get the messages "Building summary file for [folder name]... and is stucks there...

@z12: I recently had a similar problem with one of my folders in Local Folders. The folder name was "TotalEnergies", two weeks ago I saw that TB had renamed it to TotalEnergies77863ee3 and the folder was empty already the mbox file still had a size of 9.6 MB. Renaming the file in TotalEnergies didn't bring back the messages. Some user in the German TB forum having a similar problem when importing mbox files from Postbox. Eventually he could "repair" the mbox file by editing it in BBEdit and saving it as .xml file. I succeeded to repair my mbox file with the same method. Note that both files were repaired on Macs. The problem seems to be that mbox files ending with bare CR linings don't work in Thunderbird : https://bugzilla.mozilla.org/show_bug.cgi?id=1942540 "Support mbox files with bare CR line endings (produced on Mac pre Thunderbird 1.0)"

You could install BBEdit, open your recalcitrant mbox file in BBEdit and try to save it using one or another of the three line breaks as shown in my last screen shot