Enable OS X Server Messages (Jabber) account login from different domain

I have various Internet services like email, web and messages running on a Mini server running OS X on a subdomain.

Often, I want services like Messages to appear to use our main dot-com address, yet actually run on our internal dot-net domain.

So, how do I configure Messages server to appear to virtually function on our dot-com, yet still reside on a sub-domain?

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