E-mail is generally regarded as an insecure method of electronic communication for numerous reasons. Most notably, the default does not guarantee the authentic identity of either the intended sender or receiver of a message, nor does it guarantee the confidentiality and integrity of the message. While these problems can partially be addressed with commonly utilized technologies involving certificates and e-mail client plugins, current practice is insufficient for high-security applications, such as classified communications among clients of different email systems. The research presented here leverages “Trustmarks” (which were developed primarily to support efficient single sign on in a federated environment) to support secure e-mails between multiple systems where there are particularly stringent confidentiality and integrity requirements. Such a system could increase the ability of users at disparate organizations to communicate without fear that sensitive information might intentionally or accidentally be disclosed. Although there are many barriers to adoption, such a system might also eventually reduce the reliance on separate communication networks and systems for classified communications.