How do I configure sendmail?

Sendmail is a complex package with a great deal of functionality. It can be difficult for the inexperienced sysadmin to set up, and poor documentation by many system vendors doesn't help. For example, despite the long availability of the macro-based configuration tools, many support websites instruct sysadmins to hack the sendmail.cf file directly. The failure of vendors to update their support materials contributes to the perception of Sendmail as a difficult-to-configure package.