On my SBS 2003 Server, I've been trying to access OMA unsuccessfully all day. When I first checked, the OMA mapping was missing, so I recreated the Vdirs per http://support.microsoft.com/kb/883380.
At this point, OMA was returning "A System error has occurred while processing your request. Please try again. If the problem persists, contact your administrator."
I then took the following steps, all to no avail.
I tried creating entries for the server's NetBIOS name on all of its IP addresses per http://support.microsoft.com/kb/898131
I tried adding an SMTPProxy entry per http://support.microsoft.com/kb/319681.
Someone mentioned that modifying the homeMTA information was incorrect and recommended updating it manually per http://support.microsoft.com/kb/319886
I also reinstalled ASP.NET 1.1, and when all else failed, I rebooted the server.
Attempts to access /OMA via the NetBIOS name, the FQDN, and localhost all generate Event ID 1503 (MSExchangeOMA) with a (501) Not Implemented error. Attempts to connect a mobile device (I've tried a WinMo phone and an iPhone) have generated Event ID 3005.
Any insight would be greatly appreciated.