Sender Filtering

Two domain controllers are located in a single

Active Directory having two Exchange Servers

running in Member server mode. Inbound SMTP mail

from the Internet arrivers on both Exchange

servers

I configure sender filtering to reduce the

amount of junk e-mail senders in the

blocked-sender list.Users report thet they still

receive e-mail from these senders

I need to ensure that users do not receive

messages from the blocked-sender list. What

should you do on both Exchange sever'SMTP

virtual servers?  

The hint told by manager is:-

A.Enable the filter on the server'IP ADDRESS

B.Assign replay 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

I cannot understand the procedure of c, b. Do not understand why d, hence I think it is okay for a, since it shall provide more filtering.