If you implement a single sign-on solution, you are strongly recommended to maintain at least one Exchange server so that you can access Exchang SystemManager(Exchange 2003) or ExchangeManagementConsole/ExchangeManagementShell (Exchange2007 and Exchange2010) to manage mail-related attributes on the onpremises mail-enabled users.
For Exchange 2007 and Exchange 2010, the Exchange server that you maintain should have the HubTransport, ClientAccess, and Mailboxserverroles installed.