this is great!
might help to make it clear to the sender that the recipient can't "reply" and encourage them to leave contact details if they expect a response
I made a little project called pingkey https://pingkey.xyz/ it is a messaging app works like traditional mailing. You setup relays as your mailbox, people write letters and post them to you. Each letter must contain a POW which then turn into a unique POW stamp. It is that simple and I imagine it will become a alternative contact card for those people who are old enough that just hate social network and prefer to keep this simple way just like me. Feel free to try it and contact me https://www.pingkey.xyz/p/ce6232feaec4e6d01a4e00daa3648030c42017bdf589e34b53744fc49c5cba8a and give some feed back. 
Discussion
yeah, that's the mode: 1. encourage people to save their key and set up contact info if they expect a response 2. ignore if you don't care responds, eg: maybe you offer a alternative contact way in the message.