Just to note: the old folder compactor code _did_ write directly to the live DB for IMAP - it didn't take a copy to work on. Creating a separate copy of a DB can only be done for local folders at the moment - there's currently there's no way to create a standalone IMAP msgDB without the folder. I'll be working on that in Bug 1921694.
Bug 1925117 Comment 2 Edit History
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Just to note: the old folder compactor code _did_ write directly to the live DB for IMAP - it didn't take a copy to work on. There was separate compaction code for local and IMAP folders. The local version did work on a copy of the DB but the IMAP one did not. Creating a separate copy of a DB can only be done for local folders at the moment - there's currently there's no way to create a standalone IMAP msgDB without the folder. I'll be working on that in Bug 1921694.