Customers frequently ask Exchange Customer Support Services (CSS) engineers how Exchange Server organizations can be managed using multiple Web sites for Exchange Server-related virtual directories.  This is often used for segmentation purposes or for access by certain applications, apart from Web users. 

Microsoft Hosted Messaging and Collaboration (HMC) solutions are available for customers with a need to manage multiple Exchange "organizations" from the same platform.  However, many customers simply wish to use existing hardware and software without using an HMC solution.

With non-HMC solutions, Microsoft Exchange Server 2007 and Microsoft Exchange Server 2007 Service Pack 1 have some limitations in configuring Outlook Web Access and other Exchange Server-related virtual directories.  The following outlines the general supportability guidelines for Microsoft Exchange Server 2007 with and without Service Pack 1.

1. If you are using Forms-Based Authentication for your /OWA and/or legacy (/Exchange and /ExchWeb) virtual directories, Microsoft supports a single Web site per Client Access Server. Further, the /OWA and legacy virtual directories must be in the same Application Pool (AppPool).
2. If you do not use Forms-Based Authentication, you can use as many Exchange Server-related Web sites as needed for your organization. 
   However, Microsoft recommends that you use a Microsoft ISA Server 2006 server to handle Forms-Based Authentication for your various Web sites, in this circumstance.
3. If Forms-Based Authentication must be used on the Exchange Server 2007 computer *and legacy virtual directories are not used*, you can use multiple Application Pools (AppPools) for each Web site. 
   Note:  Lack of legacy virtual directories will prevent proxy to Exchange Server 2003 mailbox servers and prevent Entourage clients from synchronizing with the Exchange Server using the Exchange service. 
4. Outlook Anywhere (formerly known as RPC over HTTP) can only be set for a single Web site, per Client Access Server.  If you wish to use an approximation of a Hosted solution for Outlook Anywhere, it would have to be done using redirection per the following TechNet topic. 
   "Configure Client Access Servers": http://www.microsoft.com/technet/serviceproviders/hmc4/CMSU_HEFE_Deploy_PROC_Configure_the_Exchange_2007_Client_Access_Server.mspx?mfr=true

Troubleshooting using tracing and crash dumps to troubleshoot issues may not be conclusive in situations where multiple web sites are used in a non-HMC environment.  This may result in requests to stop the secondary web sites for verification of traces or crashes. Also, there is an additional issue around performance counters. In multiple Web site situations, they can give you unpredictable results. The reason for this is that they are not instanced on per-Web site basis and they will overwrite each other's values, making them unpredictable.

In situations where Forms-Based Authentication must be used on the Exchange Server virtual directories, and cannot be offloaded to ISA Server 2006, but multiple Web sites are required with the same or similar virtual directories, a second Client Access Server dedicated to the second Web site or an HMC solution is recommended.

- Will Duff

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