With Exchange 2007, Microsoft is wagering Exchange's continued dominance of the email market. Ironically, it's doing so by repositioning Exchange from an email/collaboration application to an unified communications platform -- in which one modality is email.
Source: Microsoft (November 2006)
To be clear, Exchange Server 2007 is not competing against other email servers. That race has been won. Rather, it is competing against a myriad of platforms that integrate multiple electronic communication (e-communication) modalities with the Outlook client.
For example, WebEx integrates the Outlook calendar and address book into its WebEx AIM Pro client.
Normally, Outlook connects to the Exchange server for email and calendaring. However, it could also connect to almost any enterprise-level email server, such as Scalix, Mirapoint and Communigate, and maintain an Outlook/Exchange "look and feel" for the end user. At the same time, some of these email servers are extending their e-communications beyond email. For example, Communigate integrates VoIP, instant messaging, video and email in one platform.
Ironically, it is the power and widespread deployment of the Outlook client that has opened a Pandora's box of e-communication competitors to Exchange.