For more than 25 years, Sendmail has served as the Internet’s primary MTA. Many applications written for Linux expect to find Sendmail running on the server. Writ- ten before the Internet became open to the public, however, Sendmail has many of the security problems listed on the Common Vulnerabilities and Exposures (CVE) list hosted at http://cve.mitre.org. Fortunately, other MTAs have emerged to take Sendmail’s place. The main problem these MTAs face is the expectation by core applications that Sendmail will be present on the Linux server. To get around this, MTAs such as Postfix and Exim must be able to appear to applications as if they are Sendmail. We call these drop-in replace- ments, and they can run in a Sendmail mode. Postfix is our preferred replacement for Sendmail. Postfix is faster than Sendmail, has a more secure, modular architecture, and offers many of the features required by a high-volume mail provider. Postfix doesn’t provide deprecated protocols, but uses the In...