Forged letters sound *awesome*, though the mechanic must be limited. Perhaps a significant chance of it being discovered as false due to most folks recognizing a Lord's seal, which isn't exactly easy to recreate. (Signatures and seals were a BIG deal back then in terms of royal and noble correspondence.)
No thanks. We've discussed forgeries and message interception many times. They are both, IMNSHO, horrible ideas. As Foundation says, they create distrust in the message system. And since the message system is really the heart of the game, it creates distrust in the game itself. People will get around it by simply asking OOC, or asking the sender "did you *really* send that?" It really won't add anything worthwhile to the game.
