|Copy the given message object into this mailbox.
Copy the given message object into this mailbox.
The message object will be one which was previously returned by
Implementations which wish to offer better performance than the default implementation should implement this interface.
If this interface is not implemented by the mailbox,
IMailboxIMAP.addMessage will be used instead.
|Either the UID of the message or a Deferred which fires with the UID when the copy finishes.