Hi, I'm talking about a POP account. Every time I empty the trash, either manually or by exiting Thunderbird, the "Trash" folder is being silently deleted,

That is how the empty trash function is designed to work in a pop account. Simply delete the file (which Thunderbird shows as a folder)

and from that moment on I cannot any longer delete messages, receiving a "not enough space" error. If exit the program, and trash is set to be emptied on exit, the next time I reopen Thunderbird there is no "Trash" (Cestino in my language, I have Thunderbird in italian) folder.

That sounds suspiciously like an antivirus issue. Have you added an exclusion to your antivirus program for the Thunderbird profile folder? Such exclusion are becoming ever more necessary.

General info: Images shows: *.mozmsgs folders These contain *.wdseml files which are a partial copy (up to 49KB) of a message in the mail folder. It is used by Windows Search Integration. They're actually renamed .eml files. You can disable this feature: Settings -> General scroll to 'System Integration' Uncheck checbox: "Allow Windows Search to search messages".

You can still search via Thunderbird either using: Global Search or Quick Filter seach or via Menu Bar - Edit > Find > Search Messages or via Menu app > Tools > Search Messages

Image shows a 'Trash.sbd' folder - what is in that folder ? It gets created when you delete a folder. But....does it contain a 'Trash' mbox file (no extension) ? If yes, Exit Thunderbird now. You cannot edit that 'Local Folders' folder in profile if Thunderbird is running - nothing will get saved.

Access 'Trash.sbd' folder and move the 'Trash' mbox file out of the 'Trash.sbd' folder and put it back next to into the 'Local Folders' folder next to Trash.msf as circled in image. If there's no 'Trash' mbox file Create a new Text Document called Trash. It will create a file called 'Trash.txt'. Then rename it to remove the .txt extension, so it becomes an mbox file.

Then delete the 'Trash.sbd' folder. I would also delete the 'Trash.mozmsgs' folder because your Trash is empty and if you really want to use Windows search instead of Thunderbird a new one will get created.

If you have an Anti-Virus product (either default used by Windows OS or one you have installed) which is allowed to scan any file that gets opened for reading/writing/updating etc or any product that can cleanup files etc eg: CCleaner etc - Advise you do not let them have access to your profile folders because if they do not like something in a file they might try to fix/quarrantine or even delete the file because they have no comprehension that the file may contin more than one email.

You say 'i'm talking about a Pop account' and then say you are looking in 'Mail/Local Folders' in profile.

Have you set up the Pop account as a deferred account more commonly called 'Global Inbox' ?

In other words - In Folder Pane, you do not see a Pop account listed with it's own set of folders - everything is directed to use the 'Local Folders' account ?

This is what I have found: In my beta version, I've set up a pop account which is a deferred account aka using Local Folders. When emptied that 'Trash' mbox file in Local Folders does not get removed.

In my current esr version, I've got pop set up as separate own account using it's own folders. When Trash is emptied whether it's the pop account or Local Folders, the 'Trash' mbox file is never deleted.

There might be an interference with some "cleaning tool", this could even be a feature of some antivirus software. It is known that certain cleaning tools remove mbox files with a size of 0 (zero) kB. I've seen this several times in forums and during remote controls that big folder/subfolders hierarchies in Local Folders have crashed because of empty mbox files being deleted by such a tool.

When you empty a Trash folder the Trash file in the profile is automatically reset to 0 kB and thus might get a prey of a cleaning tool.

@Antonello: which antivirus software are you using?