To avoid spammers using your mail server to relay their messages, you need to _______.

A.
Disable the relay control in /etc/aliases

B.
Set up a ruleset for this in /etc/sendmail.cf

C.
Set up relay control in your DNS’s MX record.

D.
Recompile sendmail with the -NORELAY flag.

Explanation:
For an overly complicated example see http://www.sendmail.org/m4/anti_spam.html#header_checks