Although message security features have been available in Microsoft Exchange since the first version, Exchange Server 2003 increased support for Secure/Multipurpose Internet Mail Extensions (S/MIME) for person-to-person message security.
Microsoft Exchange Server 2007 incorporates several features to enhance the security of your Exchange 2007 organization. By default, communication between Exchange 2007 computers is encrypted. Also by default, Secure Sockets Layer (SSL) is required on all virtual directories, and a self-signed certificate is installed.
S/MIME-based message security in Exchange Server 2003
You can read Understanding Message Security to learn about the core services of Secure/Multipurpose Internet Mail Extensions (S/MIME)-based message security, which are digital signing and message encryption. Digital certificates and public key infrastructure (PKI) make S/MIME possible. S/MIME is not a single product or technology. Rather, a fully functioning S/MIME implementation includes several different components that span multiple technologies.
Message-level security in Exchange Server 2007
Domain Security refers to the set of functionality in Microsoft Exchange Server 2007 and Microsoft Office Outlook 2007 that provides a relatively low-cost alternative to S/MIME or other message-level security solutions. The purpose of the Domain Security feature set is to provide administrators a way to manage secured message paths over the Internet with business partners. After these secured message paths are configured, messages that have successfully traveled over the secured path from an authenticated sender are displayed to users as "Domain Secured" in the Outlook and Outlook Web Access interface.