Email was developed in a free and open environment. Spam forced this environment to become less trusting and sharing. Running an open relay can result in an email server being placed on a blacklist. This prevents legitimate outgoing email from being accepted by destinations. This can also result in the termination of Internet services provided by an ISP.
Testing for an open relay is straightforward and there are free services available on the Internet that will perform these tests. All MTAs can be configured to prevent open relaying with a few simple configuration changes. This chapter covered configuring Sendmail, Postfix, Exim, and qmail to prevent them from acting as an open relay.