Blacklisting Spam domains using Sendmail

In sendmail 8.8.7 and above, you can create a deny file that will block entire domains.

/etc/mail/deny file example:

IAMASPAMMER.COM REJECT
ASP-PLATFORM.COM REJECT
SMTP.OFRSVR.COM REJECT

After creating/editing the deny file you must issue this command:
makemap hash deny
which will create a deny.db file

to create the deny.db database file which sendmail uses.