Unable to let EV outlook extension , OWA Differentiates btween internal URL and external URL

I have enterprise Vault for exchange 10 an i implement and EV to archive the PST file only
in internal use the user can see the the archive explorer and retrieve mail from exchange OWA , Outlook Client and enterprise vault OWA .
After I publish the Enterprise Vault OWA through port 443 and add this line in web.config for each CAS server <add key="EnterpriseVault_UseExternalWebAppUrl" value="true"/>
and modify the desktop policy setting "External Web Application URL" TO my external URL >
I face this problem :
if user in internal network and try to access the archive the EV-outlook add-in and internal explorer establish connection to Outside network "my external URL" that i add in External Web Application URL >
i need the EV to Differentiates between internal use and external use