Secure message delivery using a trust broker

An email security system is described that allows users within different organizations to securely send email to one another. The email security system provides a federation server on the Internet or other unsecured network accessible by each of the organizations. Each organization provides identity information to the federation server. When a sender in one organization sends a message to a recipient in another organization, the federation server provides the sender's email server with a secure token for encrypting the message to provide secure delivery over the unsecured network.