Harland,
SmarterMail allows the use of domain aliases. A domain alias is basically an alternate domain name for one that already exists in SmarterMail. For example, if the domain 'example.com' is in SmarterMail and has the user 'user@example.com' added to it, by adding a domain alias for 'example.net', emails sent to 'user@example.net' will be delivered to 'user@example.com'. That means that emails sent to either domain will end up in the same mailbox.