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Web Hosting and Cloud Computing Control Panels
Howdy,
There isn't a way built into Virtualmin for handling that, but if you're up for getting your hands dirty, it is something that can be done with some more advanced Postfix configuration.
Info on how to set that up are here in the section "Restricting what users can send mail to off-site destinations":
http://www.postfix.org/RESTRICTION_CLASS_README.html