The Client Access server role supports Microsoft Outlook Web Access, Microsoft ActiveSync clients, POP3, and IMAP4 protocols. The Client Access server role also supports Web services, such as Autodiscover, free-busy, and other Web services. You must deploy at least one Client Access server if you require support for the previous clients.

Microsoft Hosted Messaging and Collaboration version 4.5 reference architecture has two types of CAS servers. One is the above mentioned for regular client functions; another is for web-based OAB distribution.

Autodiscover is key for mailbox profile configuration and OAB download. Hosted Messaging and Collaboration has custom implementation on Autodiscover service deployment for a hosting environment, which does not require an SSL per hosting tenant. Also the Solution adds new instruction to support SRV record configuration.

Exchange ActiveSync is a Microsoft Exchange synchronization protocol that is optimized to work together with high-latency and low-bandwidth networks. Exchange ActiveSync in Exchange Server 2007 SP1 includes the following improvements for administrators and end-users:

  • Exchange ActiveSync default mailbox policy
  • Enhanced Exchange ActiveSync mailbox policy settings
  • Remote Wipe Confirmation
  • Direct Push performance enhancements
These new ActiveSync mailbox policy settings are generally only available for Windows Mobile 6.1 devices. As the devices become more generally available on the market, Microsoft will add the support to the Hosted Messaging and Collaboration Solution.

The Solution recommends two types of CAS server deployment. One for regular client access service, which should be load balanced. The other is for web-based OAB distribution. Regular client access services are removed per deployment instruction.

The above recommendation is not a mandatory requirement. It is just a piece of best practice guidance.

Exchange Server 2007 SP1 includes several performance improvements to Client Access Server (CAS) role and delivers higher scalability for OWA and Outlook Anywhere protocols.

When you use Outlook Anywhere (RPC over HTTP) in a high scale environment, there are TCP connection limits that cause limitations in the number of concurrent users who you can have on your CAS servers. The latest scalability testing indicates that pure Outlook Anywhere protocol can support between 5000-6000 concurrent users. The number of concurrent connections could be lower if multiple client access protocols are supported by a single server. The recommendation of server ratio (3:1) for MBX to CAS is specific to a particular protocol mix, profile and concurrency. Considering the protocol mix that is unique to a hosting environment, we recommend that plan for a larger number of CAS servers.

Exchange 2007 anywhere access enables users to access all messaging data wherever they are. The enhancements of Exchange 2007 in OWA and Microsoft Exchange ActiveSync® policies enhance better messaging and collaboration service. OWA is also a lower cost entry point for knowledge workers who only access email from their desktop. All of these features make OWA a more consistent user experience with the Outlook features.

For more information about the new OWA and Exchange ActiveSync features in Exchange Server 2007 SP1, see New Client Access Features in Exchange 2007 SP1.

For more information about Client Access server role, see Microsoft Exchange product document Client Access.

For more information about CAS architecture, see Understanding Proxying and Redirection.