First of all, look in the BES Admin sections - particularly for Exchange. This is the world famous "Send As" problem. Here are some articles to start with:
Delegated permissions are not available and inheritance is automatically disabled
search "AdminSDHolder, BES, DST2007" using your favorite web search engine
Users cannot send e-mail messages from a mobile device or from a shared mailbox in Exchange 2000 Server and in Exchange Server 2003 "Special rules for adminSDHolder Protected Accounts"
http://www.blackberry.com/btsc/articles/157/KB04707
Good luck - start in the BES Admin section please....