If you route traffic to your sipXpbx 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 the call.
To handle this situation, you can configure any number of domain aliases to property accept the call. From the web UI, 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.