OnionMail is an anonymous and encrypted mail server meant to run on the TOR network without losing the ability to communicate with the Internet. This ensures greater privacy for the users but also protects against eavesdropping from the NSA and from other threats. ...
To use OnionMail all you need is an email client connected to the TOR network without install any special or complicated software. (Example: Claws-Mail or Thunderbird).
All OnionMail server are connected in a federated network to check the SSL certificates and working state. All federated server can connect to the Internet via exit/enter servers to forward messages between networks.
OnionMail through the exit/enter nodes forwards messages between networks TOR and internet in a transparent manner.