If I were to design a replacement messanging system to replace email, I would design it with deny by default, where messages that aren't signed by someone on yo

If I were to design a replacement messanging system to replace email, I would design it with deny by default, where messages that aren't signed by someone on your contact list are rejected. Maybe with a system to request that someone add you to their contact list (though with a limit on how much text can be in that request). Maybe something like that could be done with email, but without a culture around it, figuring out what addresses you need to add to your contact list when you add new services could be a pain.