All of the MTAs featured here are highly configurable. If possible, test changes on a test server and not on a live MTA, and always make a backup of your configuration before making changes. It is possible that you may end up altering the configuration of the MTA in such a way that it does not operate correctly.
Sendmail is the grandfather of all MTAs. Its long heritage implies that it inherits some of the friendliness of the pre-commercial Internet. This means that older installations might relay email. Additionally, installations that were first configured with an old version but have been upgraded may also retain some configuration settings that allow them to relay email.
Sendmail is packaged by most Linux distributions, as well as HP/UX, AIX, Solaris, and other commercial UNIX-based products. Each supplier may place the configuration files in different directories. In this chapter, the default locations and file names are used when discussing configuration files...