A download method is available for HTTP offline address books on Exchange Server 2007 servers that has the Client Access Server role installed

The maximum number of public folder offline address books (OAB) that can exist in a Microsoft Exchange Server 2007 Service Pack 2 organization is limited by the architecture of public folders. Specifically, the public folder store has a limit of 32 kb for Replica IDs for the public folder hierarchy. Additionally, each OAB consumes one Replica ID, each public folder store in the Exchange Server 2007 topology consumes one Replica ID, and each Exchange admin group consumes one Replica ID.

In a hosting environment, such as Microsoft Hosted Messaging and Collaboration, each hosted tenant is typically configured to use a tenant-specific OAB if MAPI clients are used. This limits the total size of Replica IDs for hosted tenants in a single Exchange organization to less than 32 kb.

Exchange Server 2007 Service Pack 2 introduces a method to distribute OABs. This method uses HTTP downloads from a user-defined location on an Exchange Server 2007 server that has the Client Access Server (CAS) role installed. This OAB download method is available for Microsoft Office Outlook 2007. The Web download location is provided to a client by using an AutoDiscover Web service request when the client is started.

To resolve this issue, install Update Rollup 1 for Exchange 2007 Service Pack 2, and then enable the new method that is described in the "Introduction" section. To do this, follow these steps:

1. Install Update Rollup 1 for Exchange 2007 Service Pack 2. You can obtain this update from the following Microsoft Knowledge Base (KB) article:
   971534 Description of Update Rollup 1 for Exchange Server 2007 Service Pack 2
2. Set the (REG_SZ) value of OAB Extension Attribute registry entry under the following registry subkey to specify an Active Directory user object attribute on the related CAS servers:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchange Autodiscover\
   
   
   Note This user object attribute is evaluated to determine the Web download OAB location for the user. You can use any extension attribute from 1 to 15, that are not already being used.
3. Manually map the user extension attribute to the distinguishedName property of the corresponding OAB. For example, if you set "extensionAttribute2" as the value of the OAB Extension Attribute object, you must also map this attribute of the users to the distinguishedName property of the OAB.

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