In Microsoft Exchange Server 2007, Client Access server and Mailbox server supports E-mail messaging applications.
Client Access server role supports Outlook Web Access and Microsoft Exchange ActiveSync client applications, Outlook Anywhere, Microsoft Entourage 2004 and Entourage 2008 for Mac, and the Post Office Protocol version 3 (POP3) and Internet Message Access Protocol version 4rev1 (IMAP4) protocols. The Client Access server role also provides access to free/busy data by using the Availability service and enables clients that are running Microsoft Office Outlook 2007 and certain mobile operating systems to download automatic configuration settings from the Autodiscover service. For more information about these applications, see Client Access Role.
The Mailbox server role hosts mailbox and public folder databases. It also generates the offline address book. Mailbox servers provide services that calculate e-mail address policies and address lists for recipients, and enforce managed folders. For more information about Mailbox server, see Mailbox Role.