The Autodiscover service is usually enabled in a default installation of Exchange 2010/2007. However, you must configure the service so that it can send the ActiveSync server address in the Autodiscover response. Reference the Microsoft TechNet articles listed below for instructions on how to do this.
Managing the Autodiscover Service 2010
http://technet.microsoft.com/en-us/library/bb124251.aspx
Managing the Autodiscover Service 2007
http://technet.microsoft.com/en-us/library/aa995956.aspx
http://technet.microsoft.com/en-us/library/aa998277.aspx
Additional Reference:
[MS-OXDISCO]: Autodiscover HTTP Service Protocol Specification
Version 2.0
http://msdn.micrsoft.com/en-us/library/cc307725.aspx
[MS-ASCMD]: ActiveSync Command Reference Protocol Specification
Version 1.02
http://msdn.microsoft.com/en-us/library/cc307725.aspx