I’ve just updated my sendmail.mc with some RBL’s checks (dnsrbl feature) which is working out quite well. By using Sendmail with dnsrbl checks instead of a anti-spam program such as Mailscanner which also uses the same RBL mechanism you take the advantage of Sendmail dropping the connection before receiving the email. In this case you are not only reducing the amount of spam, but also the system resources and bandwidth.
Hopefully we’ll notice a slight drop in the spam stats I am collecting recently.

If this sounds interesting to you, have a look at the  cf/cf/knecht.mc file which comes with your distribution for more information.

Currently I am using the following dnsrbl checks:

FEATURE(dnsbl',dnsbl.sorbs.net', "550 Mail from " $'&{client_addr} " refused - see http://www.dnsbl.sorbs.net/"') FEATURE(dnsbl',sbl-xbl.spamhaus.org', "550 Mail from " $'&{client_addr} " refused - see http://www.spamhaus.org/sbl/"') FEATURE(dnsbl',bl.spamcop.net', "450 Mail from " $'&{client_addr} " refused - see http://spamcop.net/bl.shtml"') FEATURE(dnsbl',dnsbl.njabl.org', "550 Mail from " $'&{client_addr} " rejected - see http://njabl.org/"')