How do I ensure my Exchange 2003 server is pointing to the correct DC?

My Exchange 2003 server has been running fine for months, but suddenly I am seeing user/mail accounts not updating to follow changes made to the Active Directory, i.e.: disabled accounts still active, new accounts not showing up.

As far as I have ever known, Exchange does not maintain its own AD database, instead accessing a DC for everything. So how do I check to see that it is correctly pointing to my DC, and syncing/replicating whatever cache or database it needs to in order to function properly?
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Go to the Exchange system Manager
Right click on the Exchange server name and then go to the peroperties.
In the properties tab go to the Directory Access tab and there you can find the name of the DCs and there you can select DCs as well.
It is recommended to select the check box, automatically discover servers.
I would check into the RUS (Recipient Update Service) to see if you have all the domain controllers listed.  Also you have to check if the server you're pointing to is a global catalog.

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I would look in the app log of the exchange.  Are you seeing errors around DSAccess?  If not, you are getting good connection to DC/GCs.  The next thing to check is AD replication.  Is it possible that you are making changes and these changes are not being replicated to the DCs that exchange is using?  Use a tool like replmon to verify that you have good replication.  
percunoAuthor Commented:
Problem solved.

The issue was that automatically detect DCs was checked, and that was the cause of the problem, because there is still an old Windows 2000 DC serving DHCP until we have time to migrate that service, and it is no longer a part of the replication group; however, at some point the Exchange server decided to authenticate to this machine, due to (I assume) not being able to reach the Windows Server 2003 DCs for whatever reason.

So, we have disabled automatic DC detection and removed the old server from the list, and everything is peachy now.

percunoAuthor Commented:
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