Because the original IMAP specification doesn't include a move operation, we implement it as copy, followed by deleting the source message. Deleting messages in IMAP is a two stage process. First a message is marked as deleted, then the EXPUNGE command is issued. However, the EXPUNGE command will remove all messages in a folder marked as deleted. For a move operation, we don't want to remove other messages, and therefore won't issue the EXPUNGE command. However, if the server supports the UIDPLUS extension, we can specify which messages exactly should be expunged. So if that extension is available, we will use the UID EXPUNGE command on the source message of a move operation.
Since the EXPUNGE command removes all messages marked as deleted, K-9 Mail has a setting that controls when the command is issued (when deleting a message, when polling, manually via a menu option). Previously this setting was also used for move operations. However, that probably should have never been the case.
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