I sent a help request to my host's technical support with the same question of how to provide the email boxes to the portal based domains. I have pasted their answer below. It has worked quite well for me. I didn't know my host would allow me to set up mail service on a domain alias like this. Here is their instructions. And thanks Nina for your help.

Question: If I have a DotNetNuke installation and I add some new portals/domains to it, how do I provide email boxes for those domains? I see that my account allows my domains and my sub domains to have multiple email pop accounts, but a portal on DotNetNuke is only an alias to one of my Jodohost-hosted domains. So how do I set up email for the portal-based domain?

Answer: This is quite easy with Hsphere. First add your new domain as a domain alias, as per the below guide:http://www.psoft.net/HSdocumentation/user/configuring_domains.html#domain_aliasing