Qmailqmail (spelled completely in lower-case letters) is a " title="Mail transfer agent">mail transfer agent (MTA), a " title="Computer program">computer program for the routing and delivery of " title="E-mail">email over computer networks. qmail was written by " title="Cryptology">cryptologist " title="Daniel J. Bernstein">Daniel J. Bernstein.
qmail performs the same function as the " title="Sendmail">Sendmail and " title="Postfix">Postfix software. qmail's distinctions include its highly modular architecture, its adherence to Bernstein's rigorous philosophy of software design, and its security. To date, no " title="Software security vulnerability">security hole has been found in qmail.
qmail is often incorrectly believed to be " title="Open source">open source software, because it is free to use and distribute, and the source is publicly available. However, it is not. One of the requirements for software to be open source is that users be permitted to distribute modified versions; this is not the case with qmail.