Administrators can specify a domain's location for email delivery, choosing whether to deliver email locally or to the externally hosted domain. Furthermore, administrators have the option to override the external delivery for those users who exist on the SmarterMail server.
For example, imagine that you manage multiple mail servers. Under the same domain, some users exist locally in SmarterMail (used internally only), while others belong to a different mail server, like Exchange (used for public communication). By pointing the domain's location to the Exchange server and choosing the option to deliver locally if the user exists, you could accurately deliver that domain's email to both mail servers.