If you route traffic to your sipXpbx sipXcom under one hostname for exmple sipx.subdomain.domain.com on the inside of the network and to the world its simply sip.domainname.com then incoming INVITEs for email@example.com will cause a spiral to occur sending the message to and from to proxy until it eventually and gracefully maxes out, effectively not accepting not work and cause the system to not accept the call.
To handle this situation, you can configure any number of domain aliases to property accept the call. From the web UIAdmin GUI, navigate to System /-> Domain. From here, you can click on the "Add Alias" link has many times as you have aliases and enter each alias. You can also change the domain name from here.
You'll notice that by default the system puts in the IP address of the primary server and the host name of the primary server.