There is no Exchange Server 2006, 2007 is still in Beta.
Anti-spam and Antivirus
Built-in defenses against spam and phishing e-mail as well as new antivirus filtering and scanning capabilities provide comprehensive protection.
Confidential Messaging
New encryption features for both internal and Internet-based messages as well as enhanced Information Rights Management support help protect the confidentiality of messages in transit.
Compliance
Message transport and storage rules can control routing, encryption, and retention to meet organizational and regulatory compliance requirements.
Business Continuity
New data replication capabilities (local and clustered) keep the system highly available while limiting the need and frequency for tape backup.
Collaboration and Productivity
Enhanced calendaring, out-of-office, resource booking, and meeting scheduling capabilities simplify collaboration and increase productivity.
Unified Messaging
New Exchange Unified Messaging goes beyond e-mail to deliver more types of communications to the inbox, including faxes and voice mail, as well as offering new capabilities such as inbox access from a regular phone.
Web-Based Messaging
A rich Outlook-like experience from any Internet connected computer with a supported browser provides encrypted access to the inbox, powerful search, and the ability to view different types of attachments.
Mobile Messaging
Exchange Server ActiveSync technology delivers an improved Outlook e-mail and calendaring experience on mobile devices, while offering enhanced device and security policy control.