* The callback passed to append() will receive an additional argument (the UID of the appended message): <_integer_ >appendedUID.
* The callback passed to append()/seq.append() will receive an additional argument (the UID(s) of the copied message(s) in the destination mailbox): <_mixed_ >newUIDs. `newUIDs` can be an integer if just one message was copied, or a string for multiple messages (e.g. '100:103' or '100,125,130' or '100,200:201').
* The callback passed to copy(), move(), seq.copy(), seq.move() will receive an additional argument (the UID(s) of the copied message(s) in the destination mailbox): <_mixed_ >newUIDs. `newUIDs` can be an integer if just one message was copied, or a string for multiple messages (e.g. '100:103' or '100,125,130' or '100,200:201').