Email as backend to secure federated chat

Many of my non-tech friends are tired of sending emails. Instead why not create an app that lists latest contact names in an interface. When you click on the name/email address, you type "your chat message" which is sent by the app as an email. When the 'receiver/addressee' replies it forms the chat type conversation. This would mean 'users' with no smartphones are also part of conversation. If there is a 'large' email then a snippet can be displayed(with a link to open in a real email client). Bonus, if one can add GPG encryption - by looking up a keyserver for that email address and sending the chat (email) with GPG. Do you think this is viable?