external autodiscover URL does not work on Exchange 2010 SP1

Hi Folks,

Been struggling with getting autodiscover to work in our test lab. Lab is configured like our environment. Migrating from Exchange 2003. Have CAS role installed. OWA redirection works fine. Autodiscover internally works... I guess because of the SCP. But externally (through firewall) autodiscover does not work. I have done the following:

1. Recreated autodiscover URL
2. Set external url via Get-AutodiscoverVirtualDirectory | Set-AutodiscoverVirtualDirectory -ExternalUrl https://autodiscover.externaldomain.com/autodiscover.autodiscover.xml
3. I ran a packet sniffer and noticed that it wasn't able to find the autodiscover srv record.
4. So I added the srv record. Now able to find it
5. Sniffer shows that client gets the srv record and contacts the CAS server. But now I see "http 1.1 403 Forbidden" in the packet sniffer right after the client asks for the autodiscover path. So I added anonymous user and Everyone group on the ACL for the autodiscover virtual directory
6. I have verified that Outlook Anywhere works. It's just that you have to configure the client manually.....which not what we want :)

I am really stumped. Don't know what else to do. Any ideas will be greatly appreciated!

Thank you