As an author of a message I can delete a message I sent to an individual (yay!). However, the recipient is unable to see that there was a message that was deleted! I think the recipient MUST also be aware that a message they might have seen (worst case: acted on), is now deleted. Surreptitiously allowing the Author to delete a message and not informing the Recipient that a message has been deleted is not fair. Can cause endless concerns in the "wrong hands" :-p
This behavior can only be described as: BUG.