qmail is a mail transfer agent (MTA) that runs on Unix. It was written, starting December 1995, by Daniel J. Bernstein as a more secure replacement for the popular Sendmail program. qmail's source code is in the public domain, making qmail free software.

When first published, qmail was the first security-aware mail transport agent; since then, other security-aware MTAs have been published. The most popular predecessor to qmail, Sendmail, was not designed with security as a goal, and as a result has been a perennial target for attackers. In contrast to sendmail, qmail has a modular architecture composed of mutually untrusting components; for instance, the SMTP listener component of qmail runs with different credentials than the queue manager, or the SMTP sender. qmail was also implemented with a security-aware replacement to the C standard library, and as a result has not been vulnerable to stack and heap overflows, format string attacks, or temporary file race conditions.

When it was released, qmail was significantly faster than Sendmail, particularly for bulk mail tasks such as mailing list servers. qmail was originally designed as a way for managing large mailing lists