The Entry Tier architecture is designed to support between 5,000 and 20,000 active MAPI mailbox users. It includes both dedicated servers and servers running more than one role:

  • Two servers running Client Access roles, also using the CAS servers as the web-based OAB distribution servers
  • Two servers running Unified Messaging server roles
  • Two servers running Hub Transport services
  • Two external internet facing Hub Transport services
  • Three cluster continuous replication (CCR) clusters (a total of six physical servers)
    Customers looking for a lower-TCO, lower-SLA solution could run three local continuous replication (LCR) servers instead.
  • Two Active Directory Service Domain Controllers
  • One Public Folder OAB server for every 10,000 hosted organizations (for how to increase the maximum number of OABs on an OAB server to 10,000, see KB article945629.)
From manageability point view, Microsoft Hosted Messaging and Collaboration version 4.5 deployments now suggest the separation of the Client Access server from Unified Messaging server roles. If Unified Messaging service is provided, from scalability point view, it is OK to combine it with Client Access server. However, service providers should take additional consideration for the combination deployment.
Actual MAPI users supported by the system could be significantly higher, depending on the level of concurrency. For example, if in average, 10% concurrent MAPI users are active, the actual Entry Tire architecture can support up to two hundred thousand MAPI users (200,000*10% = 20,000).
Scalability is largely dependent on the protocol mix (for example, the percentage of Outlook Anywhere, Outlook Web Access, POP, IMAP) and the user’s usage profile. The recommendation is based on heavy usage profile.

The following figure shows the architecture for an Entry Tier deployment.