Exchange Proxy and Redirection (Exchange 2007 and 2010) Explained

Adam Farage
4 min readFeb 9, 2022


There has been a lot of discussion recently around proxy/redirection with Exchange 2007/2010 so I wanted to clear the air about what each one does, and how they work with CAS services.

What is the real definition of Proxying and Redirection?

To understand proxying and redirection, you should understand the meaning of both. Later in this article I will cover when/why proxy or redirection happens.

Proxying occurs when a CAS role sends traffic to another CAS role in particular situations (read below for more about that). For example here are two common situations:

  • CAS to CAS communication between two AD sites
  • CAS to CAS communication between Exchange 2010 and 2007 or 2003

Redirection occurs when there are two AD sites with Exchange 2010 / 2007 / 2003 and both are internet-facing.

When do I proxy or redirect a CAS connection?

This is a little more complex. The following CAS protocols / services are proxy enabled:

  • Outlook Web App (OWA) and Exchange Control Panel (ECP)
  • Exchange ActiveSync (EAS)
  • Exchange Web Services (EWS) and the availability service (part of EWS)
  • POP3 / IMAP

And then the only three services that will actually redirect are OWA/ECP and EAS.

On to the fun part..

When do I actually proxy my connections from one AD site to another?

Simple, and we will use OWA as the example.

Adam’s mailbox is in the EU AD site, which has an ExternalURL set to $NULL (no value) for OWA but the InternalURL is set to My internet-facing site is the US AD Site, and we have an ExternalURL/InternalURL of

Since OWA does have the capability of Proxy and Redirection, when the initial authentication request goes to the UAG or CAS (depending on your deployment) exchange is looking for three items from the GCS in a LDAP/RPC request, which is

  • My homeMDB (mailbox location)
  • My msExchangeVersion (the version of Exchange the user mailbox is on)
  • Authentication Token to fulfill my request

If it notices that my homeMDB is in another site, then it will query that CAS for the InternalURL and ExternalURL of the services in question (in this case, OWA). Since the ExternalURL is set to $NULL in the EU AD Site CAS, then the only option we have will be to proxy.

What occurs at this point is that the request gets proxied from the source server ( to the target server (, the CASHUB2 server will contact the Mailbox server for the appropriate data from the user’s mailbox and then proxy the connection back to the source server, which then delivers the content to the user. Beautiful thing.

Now for redirection:

In this situation, we now have an “Active / Active” configuration; both the US AD Site and the EU AD Site are internet-facing. My ExternalURL for OWA in the EU AD Site is, and my ExternalURL for OWA in the US AD Site is We will use Adam again as our example mailbox.

When Adam attempts to authenticate to the URL, the Exchange CAS (or UAG depending on your configuration) will perform the auth request along with the Mailbox Location and Version lookup. Exchange will realize that Adam’s mailbox is actually in the EU AD Site, and will send a HTTP 451 request back to the client to redirect to the proper internet-facing URL (in this situation, Once this is complete, it will either be a silent redirection or the user will have to authenticate again.

ActiveSync and OWA Virtual Directory permissions to make redirection and proxying to work?

To make sure we can successfully redirect Exchange ActiveSync and OWA requests, we need to ensure that the proper permissions are setup on the IIS Virtual Directories.

Exchange 2003 (non-internet-facing) and Exchange 2010:

In this situation, our endpoint for client access is Exchange 2010 as this is internet-facing and the most up-to-date version of Exchange. The Exchange 2003 site is not internet-facing, so the Exchange ActiveSync protocol would have to proxy from Exchange 2010 to 2003 for any users within the Exchange 2003 environment.

To allow this to work, you should enable IWA (Integrated Windows Auth) on the IIS Virtual Directory Microsoft-Server-ActiveSync on the Exchange 2003 server. This should not be done through the IIS Manager, as the System Attendant in Exchange 2003 will overwrite these changes and break proxying. You can do this through the steps available here.

Exchange 2007 (non-internet-facing) to Exchange 2007

In this situation, we have two AD sites both running Exchange 2007. One site is internet-facing and the other is not, thus if a user is within the non-internet-facing site we have to proxy the connection from the internet-facing CAS to the non-internet-facing CAS.

To allow this to work we need to enable IWA on the non-internet-facing Exchange 2007 CAS. To do this, we will need to go through IIS Manager, and the steps can be found here.

Single Sign On for OWA — Exchange 2010 SP2?

A new feature the Exchange Team at Microsoft implemented in Exchange 2010 is called silent redirection. I would recommend setting this up because you now eliminate the need for a double authentication when doing a redirection within OWA on Exchange 2010 SP2.

This does get a little lengthy, and there is an excellent blog from the Exchange Product Group located here.

Well, that’s all I have for now. Leave your questions below if you have any. Thanks for reading!

Adam F



Adam Farage

IT Leader, and Subject Matter Expert around Digital Workplace and Microsoft 365.