The Knowledge Base article KB 945629 updates the sizing guidance for Exchange Offline Address Book (OAB) servers. The current limit is raised to a new per-server maximum of 10,000 OABs. This ten-fold increase compared to Hosted Messaging and Collaboration version 4.0 allows service providers to save on hardware, software licensing, and maintenance cost. After raising the limit in an environment, service providers should monitor Active Directory performance carefully to ensure the change has no negative impact. Service providers should also ensure the existing hardware for the OAB servers are capable of handling the increase to ensure that customers are still able to quickly download updates to their OABs, even during peak working hours.

• Available servers that can host Exchange OABs.
  
• The capacity of servers in the OAB resource pool.
  
• OABs that have been allocated.
  
• The customer organization that owns each allocated OAB.
  

The Exchange OAB Resource Manager provides an API that tracks and allocates available servers that can hold OABs. The Exchange OAB Resource Manager can track the following:

• Available servers that can host Exchange OABs.
  
• The capacity of servers in the OAB resource pool.
  
• OABs that have been allocated.
  
• The customer organization that owns each allocated OAB.