To send and receive messages with Mailchain on Ethereum, you will need to add an existing account (or create a new one and save the private key).
mailchain account add --protocol=ethereum --private-key=[PRIVATE-KEY-VALUE] --key-type=secp256k1
--private-keyof your account to be able to sign and decrypt messages.
--key-type=secp256k1 (for Ethereum).
By default, Mailchain comes preconfigured with some default values to make it easier to get started. Etherscan is configured by default, but Etherscan recently made it a requirement to use an API key to access the API.
To obtain an API key, follow the instructions here: https://etherscan.io/apis.
To add the API key to the Mailchain Client, follow the instructions here: Mailchain Settings: Viewing and Updating