To send and receive messages with Mailchain on Substrate, you will need to add an existing account (or create a new one and save the raw seed).
mailchain account add --protocol=substrate --private-key=[PRIVATE-KEY-VALUE] --key-type=sr25519
--private-key is the raw seed of your account to be able to sign and decrypt messages.
ed25519 (for Substrate). The account type is selected when you create a substrate account.