From BlackBerry Limited:
BlackBerry Enterprise Server Express provides advanced BlackBerry smartphone features with no additional software or user license fees, and works with any Internet-enabled BlackBerry data plan or a BlackBerry enterprise data plan.
From connecting to business applications to synchronizing email, calendar entries, contacts, tasks and memos, BlackBerry Enterprise Server Express uses push technology to help deliver messages and information to BlackBerry smartphones in seconds. This helps ensure that the information on a user's BlackBerry smartphone is automatically updated with the information on their computer. Businesses with and existing Microsoft Exchange Server or Windows Small Business Server can easily get started with BlackBerry Enterprise Server Express.