You have an Exchange Server 2007 environment. You have a distribution list that contains all employees. You need to ensure that only external senders are unable to send e-mail to this distribution list. What should you do?

A.
Modify the rejected senders list in Active Directory by using Active Directory Users and Computers.

B.
Modify the message delivery restrictions to require authentication on the distribution list.

C.
Modify the settings on your Hub Transport server to require authentication for SMTP transport.

D.
Modify the settings on your Edge Transport server to require authentication for SMTP transport.