Sort of. Briar can do private messaging. However, Briar also supports Blogs and Forms. The neat part is that it is totally decentralized with traffic going over Bluetooth, WiFi and Tor. It is designed to communication in situations where an advanced adversary is targeting a group. It can be used a protests and such.

The downside with Briar is that it requires that something is online always. You can setup a old phone as a mailbox but it is still inconvenient.