Applications using MAPI or CDO may have problems if upgrading to Exchange Server 2007:
"Starting with Exchange 2007 Beta 2, neither the Messaging API (MAPI) client libraries nor CDO 1.2.1 will be provided as a part of the install of the product. As a result, there is functionality missing that many applications depend on. Microsoft Exchange MAPI and CDO 1.2.1 provide access to these APIs, which, in turn, provide access to the contents of MAPI stores."
Applications using any of the Chilkat email components (IMAP, POP3, SMTP) will not be affected because the Chilkat libraries communicate directly with the server via TCP/IP sockets and do not use MAPI or CDO.