You can use something called smtproutes to have the interworx server always use your external smtp server when sending messages FROM the interworx server to the outside world.
This would make the interworx server(s) "relay" their messages through your external smtp server. Your external smtp server would have to be configured to allow the interworx server(s) to relay through it for this to work.
To do this, you would edit the file:
If your external smtp server is "mail.domain.com" you would put, inside the smtproutes file,
Then restart the smtp service.