Exchange Server 5.5 uses RPCs to communicate within a single site, Exchange Server 2000 uses SMTP as server-to-server transport protocol. SMTP increases flexibility by not requiring a high-speed permanent network connection. Exchange 2000 Server supports <i>mixed mode</i> operation which allows it to coexist with previous versions of Exchange and facilitate directory replication with Active Directory Connector (ADC) and Site Replication Service (SRS). The default when installed is mixed mode. You can also use connectors installed on Exchange 5.5 if you operate your 2k Exchange in mixed mode. Exchange 5.5 Connectors: Professional Office Systems (PROFS) System Network Architecture Distributed Systems (SNADS) Exchange 2000 Connectors: SMTP and X.400 Microsoft Mail Lotus cc:Mail Lotus Notes Novell GroupWise To connect Exchange 2000 to PROFS, create a connector to 5.5 and a connector from 5.5 to PROFS. Client Platforms: Messaging Application Programming Interface (MAPI)-based (Exchange) Internet-based (POP3,IMAP4,HTTP,NNTP,MIME) HTTP is implemented via the Web Storage System and Outlook Web Access. The first installation of Exchange 2000 Server will take an extended time to install because it must extend the AD schema, all subsequent installs will be faster. When you connect with Outlook to look up recipient info from Exchange 2000, the information actually comes from the global catalog, not exchange 2000. Older Outlook and other MAPI-based clients are not aware of a global catalog and expect Exchange to provide this information. Earlier versions of Exchange provided their own directory service. To support MAPI-based clients, Exchange 2000 provides a DSProxy. DSProxy forwards directory lookups of MAPI clients straight to a global catalog server. MAPI-based clients obtain access to AD by using the Name Service Provider Interface (NSPI). The DSProxy process is part of the Exchange System Attendant service, and is implemented in dsproxy.dll, which resides in the \program files\exchsrvr\bin folder. To filter for these events in event viewer, in Application Log, under Event Source, select MSExchangeSA and under catagory select NSPI Proxy. To statically map a GC server for MAPI clients, the registry change is: HKLM\System\CurrentControlSet\Services\MSExchangeSA\Parameters REG_SZ: NSPI Target Server value: gcsrv.domain.com DSProxy, especially its Director Service Referral (RFR) Interface, also has the ability to divert smart MAPI clients such as Outlook 2000 to Global Catalog servers. Outlook 2000 only needs to learn that it should contact a GC server. To make DSProxy divert smart MAPI clients, the registry change is: HKLM\System\CurrentControlSet\Services\MSExchangeSA\Parameters REG_SZ: RFR Target Server value: gcsrv.domain.com With Exchange 2000, you should have at least 2 global catalog servers per site. If the site spans more than one domain, you should have another gc for each domain also.