The MsExchangeTeam explains to us how to implement TLS between two Exchange Server 2003 organizations.
We have had some requests to talk more about the process to implement TLS between two Exchange organizations. Here it is! The following walk-through should help you do this. Note that those instructions apply to Exchange Server 2000/2003. We will follow up with Exchange Server 2007 functionality at the later time.
Read more at source: http://msexchangeteam.com/archive/2006/10/04/429090.aspx