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The definition of the word expunge is
1. to delete or erase; blot out; obliterate
2. to wipe out or destroy
That pretty much explains it.
When you delete or move mails from a folder (fx INBOX) the actual mails are still there within the folder-file but now marked for deletion.
When you expunge or compact a folder it is rewritten to a new folder WITHOUT the deleted /moved mails
The old folder is then deleted and the new is given the old name.
There is no way to reverse this. But for a mail to be expunged it has to be either deleted or moved. And you have to set it up in