Exchange 2010 CAS Arrays are a great high availability feature, but are organizations making the best use of them?

One of the best practices when deploying Client Access Server Arrays is to establish the CAS Array first before configuring mailbox databases in the same site. Or at the very least, making sure you update each existing mailbox database with the correct “RPCClientAccessServer” attribute before migrating users to them.

The advantage of this is that Outlook profiles get automatically configured with the name of the CAS Array rather than the real name of a Client Access server in that site.

Considering the benefits of this, such as:

  • making it easy to scale out the CAS Array name to multiple Exchange 2010 Client Access servers
  • making it simpler to replace a Client Access server with a new one of a different name
  • migrating the MAPI endpoint to future versions of Exchange Server

…does it make it a sensible best practice to always configure CAS Arrays for AD Sites that contain Exchange 2010 mailbox servers, even when only one Client Access server is deployed?

Update: Microsoft now recommends that you always create a CAS Array in each site, even if you have only one Client Access server in the site.

Please give your opinion by voting in this poll. If you wish to expand on your vote in more detail please leave a comment below.

[poll id=”5″]

About the Author

Paul Cunningham

Paul is a former Microsoft MVP for Office Apps and Services. He works as a consultant, writer, and trainer specializing in Office 365 and Exchange Server. Paul no longer writes for


  1. Hamid

    Hi all

    I have a little question regarding CAS Array. Well, i m actually working on a migration project (E2003–>E2010 SP3). The target E2010 architecture is based on a centric Highly Available infrastructure (very basic, 2 x Multiroles servers with a load balancer).

    The client has also something like 80 Active Directory Sites, and 79 of them only containing outlook users connecting to the primary site.

    So the question is : should i create only one CAS Array in the Exchange site or have i to create one per site ?

    Thanks in advance


    1. Kanta Prasad

      One CAS Array Per site is Recommended to avoid unseen issues on client connectivity.

      Kanta Prasad

  2. amit

    Paul ,

    Can Exchange Hub server installed for different geographical locations.

    1] Single forest single domain architecture.
    2] 4 geographical locations with different smtp domains.
    3] Respective location mx will be pointed to respective location Edge server
    4] SITE A will have smtp address abcd.,com abcd mails will be delivered to SITE A EDGE SERVER.
    5] SITE B will have smtp address XYZ.COM mails will be delivered to SITE B EDGE SERVER.

    1. Avatar photo
      Paul Cunningham

      Yes, you can place different Edge and Hub servers in any location you want and use DNS/MX records to control where the email for each domain goes.

      1. amit

        Thanks Paul

        But can we have different urls for different sites

        1. Avatar photo
          Paul Cunningham

          Yes you can specify different ExternalURLs for services like OWA and ActiveSync on the internet-facing Client Access servers at each region.

        2. amit

          Paul last question

          Means we can have different georgraphic location with EDGE /HUB /CAS MBX IN HA

          MX for respective sites will be pointed to respective Edge servers.

          EX: Mumbai Russia .Holland

          Mumbai : will have different owa url smtp domain will be different .
          Russia : will have different owa url smtp domain will be different
          Holland: will have different owa url smtp domain will be different

          NO failovers for respective locations users should access their respected owa /active sync

          Hence will i require different certificates .

  3. gene campbell

    Hi Paul,
    I have a problem with my exchange 2010 with the MAPI and ACS.
    Do you have time to answer some questions

    Thank You

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