If you wish to be even more aggressive with your spam filtering, you can configure Sendmail to completely ignore senders that have bad reputations. With this step, Sendmail won’t even talk to them.
Editing the file /etc/mail/sendmail.mc, insert the following lines anywhere in the FEATURE section of the file:
dnl # Here are Sharky's favorite DNSBL definitions.
Apply the changes by saving the file and running the following commands:
/sbin/service sendmail restart
From this point on, every time an SMTP client connects to Sendmail, Sendmail will refer to the blacklist authorities you added to verify the client’s reputation. If the client is reported to have a shady reputation, Sendmail will hang up on him.