What should you do on both Exchange servers’ SMTP virtual servers?

You are the Exchange administrator for your company. Exchange Server 2003 runs on two Microsoft Windows Server 2003 member servers.
The company’s network consists of a single Active Directory domain.
Two domain controllers are located in a single Active Directory site. Inbound SMTP mail from the Internet arrives on both Exchange servers.
You configure sender filtering to reduce the amount of junk e-mail that is received by company users.
You specify a list of known junk e-mail senders in the blocked-sender list, users report that they still receive e-mail from these senders.
You need to ensure that users do not receive messages from the blocked-sender list.
What should you do on both Exchange servers’ SMTP virtual servers?

A.
Enable the filter on the servers’ IP addresses.

B.
Assign relay permissions to only authenticated users.

C.
Configure the servers’ authentication settings to resolve anonymous e-mail.

D.
Configure the servers to perform reverse DNS resolution on incoming messages.

Explanation:
The filter is created, but has not been applied. Hence, the junk mail still arrives.
The incorrect answers:
B: Assigning relay permissions is helpful to avoid Denial of Service (DoS) attacks, but would not affect the delivery of inbound spam messages. Also does not apply the given filter anywhere.
C: By default all incoming mail, whether spam or not, is authenticated anonymously. Resolving these names would incur significant overhead, and many times would block even valid email. This also does not utilize the given filter.
D: Configuring servers to perform DNS resolution on incoming messages would not prevent spam, and certainly wouldnot take into consideration the filter that was defined.