Exchange 2000/2003 Native Mode / Mixed Mode

To take advantage of all the new Exchange 2003 features you need to change
from mixed to native mode. In mixed mode Exchange 5.5 and Exchange 2003 will
be able to communicate and co-exist in the same organization. Exchange 5.5
servers do not understand some of the new features of Exchange 2003, so you
must not switch to native mode until you de-commission the last Exchange 5.5
server. After switching to Exchange native mode, you can remove all ADCs.

New Exchange Server 5.0 or 5.5 computers cannot be introduced into the
organization once the functional level is raised to native mode.

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The following are some of the features of Exchange 2000 Server or Exchange
Server 2003 native mode:

	o Rename your Exchange organization.
	o Routing groups can consist of servers from multiple administrative
	  groups.
	o Move mailboxes between servers in different administrative groups.
	o Create Query based Distribution groups (LDAP queries).
	o Create the InetOrgPerson object (See more here).
	o Routing groups can contain servers from multiple administration
	  groups.
	o Simple Mail Transfer Protocol (SMTP) is the default routing
	  protocol.
	o Routing bridgehead server pairs use 8BITMIME data transfers giving
	  significant bandwidth improvement over routing group connectors.
	o Servers can be moved between routing groups. 
	o The InetOrgPerson object class can be mailbox/mail-enabled.
	o The Information Store service in Exchange Server 2003 automatically
	  ignores and removes "zombie" Access Control Entries (ACEs) from the
	  Exchange Server 5.5 computers that were used in an organization. 
	o In Native mode there is no need for any ADC' agreements with NT 4.0,
	  so they can safely be removed.

Note that servers cannot be moved between administrative groups in either
mixed mode or in native mode.

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Switch Exchange 2003 to Native Mode

Mixed mode is the default when you install the first Exchange 2003 server.
To switch to native mode: open the Exchange 2003 Administration program, then
right-click the Organization Name, Properties and select Change Operation Mode.
To work around rebooting, restart the Microsoft Exchange Information Store. You
may want to restart the other Exchange services at this point also.

Exchange 2000 Co-Existing with Exchange 2003

If your situation is Exchange 2000 co-existing with Exchange 2003 but without
any Exchange 5.5 servers, it is safe to switch to Exchange native mode.

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Description of mixed mode and native mode in Exchange 2000 Server and in
Exchange Server 2003
http://support.microsoft.com/kb/270143/

XADM: Preparing a Mixed Mode Organization for Conversion to Native Mode
http://support.microsoft.com/kb/272314/

How To Convert from Mixed Mode to Native Mode in Exchange
http://support.microsoft.com/kb/327779/