Mailchain claims to be a Multi-Blockchain Messaging Application. Send and receive messages between blockchain addresses & accounts through a simple, email-like interface.

Mailchain makes it possible to easily send and receive rich-media HTML messages between blockchain public addresses through a simple, email-like interface.

All message contents & attachments are encrypted so only the intended recipient of the message can decrypt and view messages. Messages are stored off-chain to minimise the data added to the blockchain.