I would think long an deep before implementing it, though. It opens up a can of worms. If it works, which depends on the receiving side, OTRS will receive an answer to your agent's mail. As OTRS does not have much of a chance to know it's just the "has been received" notification (they look too different with different mail clients and different native language settings), the agent will be informed of a client's answer. Usually you only want such notification for human-sent replies. Those computer-sent replies will either result in your agents spending a lot of time coming back to tickets just because such recieved notifications came in, or it will result in your agents ignoring OTRS's notifications about a customer's reply, expecting it to be yet another received notification.
Yes, exactly my point. Anything that relies on the receiving system to interpret and respond to such a request is pretty much useless for the purpose -- there's no reliable standard to indicate what such a response should look like, and no leverage to force system updates to implement such a thing even if it did exist. The extra noise in the tickets will just cause important messages to get lost in the noise for no visible gain.