SmarterMail Professional edition has all of the features required to host a fully-functional mail server.
The Enterprise edition offers additional features, including:

   Advanced Collaboration (Groupware) - Shared calendars, contacts, tasks, and notes.
   LDAP Support.
   Global Address List (GAL).
   Message Prioritization and Throttling.
   Active Directory Authentication.
   System-wide Email Archiving.
   Over-the-Air synchronization for BlackBerry, iPhone, and Windows Mobile devices.
   SmarterMail Sync for Microsoft Outlook.
   SmarterMail Sync for SmartPhones and Windows Mobile devices.
   Custom and Emailed reports.