Most people think of electronic mail as the program that they interact with—their mail client, technically known as a Mail User Agent (MUA). But another important part of electronic mail is the software that actually transfers the mail from the sender to the recipient—the Mail Transfer Agent (MTA). The first MTA on the Internet, and still the most prevalent, was sendmail. Sendmail was first created before the Internet officially existed. It has been extraordinarily successful, having grown fro...