--network=mainnet or a testnet you are working on.
--private-keyof your account to be able to sign and decrypt messages.
--key-type=secp256k1 (for Ethereum).
Mailchain stores your private key in an encrypted format, a passphrase is required.
Enter a non-guessable passphrase, and again to confirm it. You must remember your password to decrypt the key later to send or receive messages.
You should never expose your private keys to an entity you don't trust.
Mailchain stores your private key locally, in an encrypted format. Your private keys are never sent anywhere. They are only used to decrypt messages data, and sign transactions.
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.