Navigation:  Exchange Server Toolbox > Configuration > Spam filter options > Black- / Whitelist

 

Specify a mail address or domains of senders which should be treated as spam (Blacklist) or good mail (Whitelist).

 

You can use the 'Add Active Directory' button to import all mail addresses from your Active-Directory to the Whitelist. You should only do this if your are using your Exchange Server "from outside" as relay or SMTP server.