Hello,
Create an "A name" on your external DNS.. for example ; archive.acme.com
Then add the below lines in web.config on CAS Server
<add key="EnterpriseVault_ExternalWebAppUrl" value="archive.acme.com/enterprisevault"/>
<add key="EnterpriseVault_UseExternalWebAppUrl" value="true"/>
If you are not able to create an "A name" on external DNS, create a redirection rule on you firewall based on "header". That rull will redirect the "your-owa-addres.com/enterprisevault" to Enterprise Vault local IP..
Also check this,
http://seer.entsupport.symantec.com/docs/308450.htm