We recently upgraded from Microsoft Exchange 2003 to Exchange 2007. After migrating all of the mailboxes and public folders we successfully brought the Exchange 2003 server offline.
The problem started when we needed to add new users to the system. I have performed some troubleshooting and here is what I found.
1. If user's AD account is added through Active Directory Users and Computers and the users mailbox is setup using the Exchange 2007 Management Console the user does not show up in the Global Address List.
2. If the user's AD account and Mailbox are created through the Exchange Management Console the user is successfully added to the Global Address List.
The Exchange 2003 Management Tools have been removed from Active Directory Users and Computers on the machine that we used for the tests.
On another note, we needed to bring the Public Folder Store on our Exchange 2003 server because Free/Busy Data was not being shown on Outlook 2003 Clients. The Message Store is still offline.