Qmail
qmail (spelled completely in lower-case letters) is a mail transfer agent (MTA), a computer program for the routing and delivery of email over computer networks, written by cryptologist Daniel J. Bernstein. Known for its simple structure and robust design, it has not suffered from any known security hole to date.qmail is often believed to be open source, because although it is free to use and distribute, and the source is publically available, distribution of modified versions is not allowed.
