Here is what we did and why it did not work so well:
To make the new employees show up in the Global Address List (GAL), we created them as contacts. We gave them an email that matches our domain which forwards to their own domain. All our current employees can exchange email with the new employees.
However, during migration these contacts must be deleted, and mailbox-enabled users must be created in order to use the same email addresses, etc. The problem is that Microsoft Outlook and Entourage cache the email addresses. If a person replies to or sends a new message to a new employee, it tries to deliver the message to the contact. Since the contact is no longer there, the message bounces.
I've recommended that we migrate the users after hours on a Friday evening so we have the weekend to have a new address book pushed down to the client. If we move them during the week, we'll receive too many calls to our call center. To get around the problem, we can delete the cached address book and set the client to On-Line mode. This is an ugly solution.
As an administrator, I'd like to find a solution that is totally transparent to my end users. Do you have any suggestions?
Read more about this issue in the Microsoft articles "Enabling replies to old organization after Move Server Wizard" and "Cannot reply to messages that are sent from a user account that was moved to a different site."
You should also take a look at the ADModify.Net utility. It's very useful because it allows you to make bulk changes in Active Directory. Microsoft provides more information on ADModify.Net.
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