Summary: A simple way to send messages confidentially, without the sender's name being attached. So the recipient will not be aware who has sent the letter to them.
Details: A way to enable more methods of subterfuge, backstabbing and various more nefarious tasks. The cost to send a letter like this confidentially would have to be substantial enough that it's not misused, so I'd say it should cost 1 honour per use, as you're engaging in back alley dealings this would make sense from a role play perspective.
Benefits: The advantage to such a system, would mean there would be slightly more options for players to act against their own realm in preparation for betrayals, open up better possibilities to recruit for the already under utilised Secret Societies, and to be able to leak information to certain people. Or, even to send anonymous love letters!
Negatives: The downsides, I can't imagine there are many, other than potentially encouraging more use of subterfuge, but that being a downside is subjective. An exploit would perhaps be if someone intentionally wanted to tank their honour this would be a good way, but there aren't any reasons to do that, that I know of.
