qmail is a modern SMTP server which makes sendmail obsolete, written by Dan Bernstein, who also has a web page for qmail. qmail is a secure package. You can download netqmail 1.05 (Redhat RPMs, and Debian .debs, HP-UX, Gentoo, and OpenBSD ports) and redistribute qmail for free. You can get the "big picture" of how qmail is organized.