Selectively Rejecting SPAM Using Sendmail

With the growing popularity of the Internet, unsolicited electronic mail (spam) has become a major concern. It fills up user’s mailboxes, clogs mail relays, wastes postmaster time, and creates ill will for sites that have been used as a relay. Most sites want to filter spam before they receive it but filtering spam is hard to do without filtering legitimate mail messages. This paper discusses what characterizes spam and describes rulesets that can be added to a sendmail version 8.8 [1] sendmail.cf file to selectively reject mail from specific addresses, domains or IP addresses and to prevent spammers from relaying mail through a site. It discusses the different issues facing corporate sites and the special issues facing Internet Service Providers (ISP). The rulesets presented have been implemented as M4 template files so they can be easily integrated into a sendmail 8.8 sendmail.cf file as a FEATURE using M4. These rulesets are currently in use at Harker Systems and other sites, and are available via anonymous ftp.