Background

This article assumes that your Exchange Autodiscover is setup and working correctly internally.

For external connections MS Exchange Autodiscover uses following 4 methods to retrieve automatic email client configuration:

1. Connecting to Autodiscover URL: https://domain.com/AutoDiscover/AutoDiscover.xml
2. Connecting to Autodiscover URL: https://autodiscover.domain.com/AutoDiscover/AutoDiscover.xml
   
3. Connecting to Autodiscover redirect URL and looking for Redirection to HTTPS Autodiscover at: http://autodiscover.domain.com/autodiscover/autodiscover.xml
   
4. Looking for DNS SRV record
   

First two methods work fine for single SMTP namespace organisations. Last two can deal with multiple SMTP namespaces. In this example I will use third, the Redirect method.

Configuring Exchange Autodiscover with HTTP redirect method

1. Login to your SMTP domain management console and add A type record autodiscover and point it to an external IP address that is not used for any HTTP service.
   i.e. autodiscover.domain.com      A      86.222.51.5
   Repeat this for your other SMTP domains.
   
2. Login to your Exchange Client Access Server and add a new internal IP address.
   i.e. 192.168.10.11
   
3. Login to your firewall and setup incoming NAT redirection to forward HTTP traffic from 86.222.51.5 to 192.168.10.11.
   
4. Login to Exchange Client Access Server and open IIS Manager
   
1. Create a new web site autodiscover-redirect (name is not important) and Bind it to the IP address 192.168.10.11.
   
2. Inside autodiscover-redirect website create new virtual directory called autodiscover
   
3. Inside autodiscover virtual directory create a blank file autodiscover.xml
   
   
4. Select autodiscover.xml and click on HTTP redirect.
   Configure as per screenshot below. companymail.com is your Exchange external access domain with a valid SSL certificate.
   
   
5. Go to https://testconnectivity.microsoft.com and perform a test. Autodiscover should succeed using the third method (Redirect)
   
   

Windows Server 2008 R2
Exchange Server 2010