Log Name: Application
Source: MSExchange Web Services
Event ID: 16
Task Category: Core
Client Access server Internet-Facing-CAS-Name failed to proxy Exchange Web Services to Active Directory site CN=Default-First-Site-Name,CN=Sites,CN=Configuration,DC=yourdomain,DC=tld because there are no applicable Client Access servers in the destination site. Please check the configuration of the servers in site [No Server].
If you have an Exchange Server 2010 infrastructure that contains more than 1 Client Access Server (CAS) with 1 Internet facing and the rest are not, then you may see this error message in application log in Internet Facing CAS server. The problem is that the Internet Facing CAS proxies the request for Exchange Web Services (EWS) to the Non-Internet facing CAS. It is failing because the servers are not trusting the certificate. CAS-CAS proxy problem in Exchange 2007 was fixed by Service Pack, so it may be fixed in the future patches by Microsoft.
You must make a registry configuration change on the Client Access server that receives the proxy requests. Do the following:
1- Open the Registry (Start> Run > Regedit)
2- Browse to HKEY_LOCAL_MACHINE/System/CurrentControlSet/Services/MSExchange OWA/
3- Right Click MSExchange OWA, then New > Key
4- Type in AllowInternalUntrustedCerts
5- On the right-hand side set the Default value to 1
6- Close the registry editor
7- Reboot your CAS server in order to take effect