In this post the MsExchangeTeam.com explain to us how to troubleshoot TLS issues.
Understanding how certificates are selected for a Transport layer Security (TLS) session will help you troubleshoot TLS issues. Since we shipped Exchange 2007, support engineers Jenny Frye and Stuart Presley have been helping customers work through issues around deploying Domain Security and using TLS to connect Hub Transport servers and Edge Transport servers and to enable POP and IMAP clients to encrypt traffic with Hub servers.
Read more at: http://msexchangeteam.com/archive/2007/05/03/438266.aspx