Error message when you try to install the CAS role on a server that is running Exchange Server 2007:

In an organization that runs both Microsoft Exchange Server 2003 and Microsoft Exchange Server 2007, you may be unable to install the Client Access Server (CAS) role on a server that is running Exchange Server 2007. Additionally, you may receive the following error message:

This problem occurs because the Enable-OABWebDistribution task fails during CAS Setup. This failure occurs if the following conditions are true:

�	You created the default Offline Address Book (OAB) for the organization on a server that is running Exchange Server 2003.
�	You used Exchange Server Manager to move the default OAB to a server that is running Exchange Server 2007.

This operation does not populate the msExchVersion attribute.

To resolve this problem, use the ADSIEdit tool to manually populate the msExchVersion attribute. To do this, follow these steps:

1.	Locate the Active Directory directory service object for the OAB in the following location: Configuration, CN=Services, CN=Microsoft Exchange, CN=organization, CN=Address Lists Container, CN=Offline Address Lists
2.	Obtain the properties for the OAB that was moved.
3.	Set the value of the msExchVersion attribute to 4535486012416.
4.	Restart the installation process for Exchange Server 2007 to install the CAS role.

This problem is resolved in Exchange Server 2007 Service Pack 1 (SP1). However, if you install Exchange Server 2007 SP1 on a server that is running Exchange Server 2007 and that has already had the CAS role installed, this problem still occurs. Exchange Server 2007 SP1 has some code logic changes to handle the versioning problems of the OAB. However, this logic runs only during the installation process. The status of the OAB is not rechecked after you install the CAS role.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.