SmarterMail Enterprise edition

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.