MsExchangeTeam has published a blog entry about Certificates and Exchange Server 2007. They used a scenario to provide more information about certificates in a simple environment.
Exchange is now using certificates for more than just web, POP3, or IMAP. In addition to securing web services, Exchange 2007 has also incorporated Transport Layer Security (TLS) for session based authentication and encryption. TLS has been around for a while but the Exchange 2007 implementation is able to mutually authenticate with external transport servers, internal servers and Outlook 2007 clients. TLS as implemented in Exchange 2007 is an effective alternative to S/MIME and is much easier to administer. 'Domain Security in Exchange 2007' is an excellent whitepaper on Exchange 2007 and TLS so we aren't going to reproduce it in this post, but we wanted to give mention to this new change and point everyone in a direction to read more on this topic. You can download it here: http://technet.microsoft.com/en-us/library/bb266978.aspx
Read at source: http://msexchangeteam.com/archive/2007/07/02/445698.aspx