EthMail is a whisper / ethereum mailing system.
It requires your node to have whisper enabled (--shh --rpc --rpcapi "db,eth,net,web3,shh" --rpccorsdomain "http://ethmail.io" or just use mist which works out of the box). Please note that the whisper messages are extremely short lived (for now) and you will loose messages if you're not online.
Please also unlock the account from which you'd like to send messages. It's required by eth.sign which proofs messages authenticity.