On Redhad, edit /etc/mail/sendmail.mc and add the following line towards the end of the file before the first MAILER line:
FEATURE(`dnsbl', `sbl.spamhaus.org',`"Rejected due to Spamhaus listing see http://www.abuse.net/sbl.phtml?IP=" $&{clientaddr} " for more information"')dnl
Then run:
cd /etc/mail/ make /sbin/service sendmail restart
