Qmail

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 alternative to the popular Sendmail program. Originally license-free software, qmail's source code was later dedicated in the public domain by the author. 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 from 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. At the time of qmail's introduction, Sendmail configuration was notoriously complex, while qmail was simple to configure and deploy. qmail encourages the use of several innovations in mail (some originated by Bernstein, others not): Maildir Bernstein invented the Maildir format for qmail, which splits individual email messages into separate files. Unlike the de facto standard mbox format, which stored all messages in a single file, Maildir avoids many locking and concurrency problems, and can safely be provisioned over . qmail also delivers to mbox mailboxes. Wildcard mailboxes qmail introduced the concept of user-controlled wildcards. Out of the box, mail addressed to "user-wildcard" on qmail hosts is delivered to separate mailboxes, allowing users to publish multiple mail addresses for mailing lists and spam management.