Mailbox servers perform the following functions:
Host mailbox databases
Provide e-mail storage
Host public folder databases
Calculate e-mail address policies
Generate address lists and offline address books (OABs)
Conduct Multi-Mailbox Searches
Provide high availability and site resiliency
Provide content indexing
Provide messaging records management (MRM) and retention policies
The Client Access server role is one of five distinct server roles for Microsoft Exchange Server 2010. It supports the Outlook Web App and Microsoft Exchange ActiveSync client applications, 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 certain clients to download automatic configuration settings from the Autodiscover service.
The Client Access server role accepts connections to your Exchange 2010 server from different clients. Software clients such as Microsoft Outlook Express and Eudora use POP3 or IMAP4 connections to communicate with the Exchange server. Hardware clients, such as mobile phones, use ActiveSync, POP3, or IMAP4 to communicate with the Exchange server. You must install the Client Access server role in every Exchange organization and every Active Directory site that has the Mailbox server role installed.
