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I know DNN allows you to host multiply domains (portals) from one instance. One of the downsides to this is setting up email for those different domains. For example with PowerDNN you are going to register the site to a specific domain. Say www.123.com and the email they have for that domain would work fine. But say I have a friend who I build a site for and host it as a separate portal at www.456.com. How do people handle providing an email service for a secondary portal so the user of that portal can act independently of the main site in terms of email usage.