Outlook 2010 connecting Remote CAS servers in Exchange 2010.

Hello, 

We are multi-domain and multi site Exchange 2003/2007 / 2010 Set up. 

One  of our user sits in Latin America and his mailbox hosted in US Data center . when he launches the his Outlook 2010 , he getting Certificate pop-up from my APAC CAS servers which self signed. 

When i run Test-Outlook configuration from his outlook , it first searches the APAC servers and then return to my US CAS servers. 

I checked the user AD site and it is added in my US CAS server AutoDiscoverSiteScope , but still this same behaviour in his outlook. 

then our Exchange 2003 users who uses Outlook 2010 and his mailbox hosted in Latin America getting certificate  pop up from US CAS servers , who's AD-site is not added in my CAS server AutoDiscoverSiteScope as they in Exchange 2003.

Please help me how to fix this ?

Regards

K Gopi 

Regards Gopi K