Compare Exim vs. Postfix vs. qmail

Exim serves as a mail transfer agent (MTA) for Unix-like operating systems. Initially developed by Philip Hazel in 1995 for the University of Cambridge Computing Service's email systems, Exim is released under the GPL, allowing users to freely download, utilize, and modify it. While it shares some similarities with Smail 3, Exim has evolved to offer greater user-friendliness and flexibility. Both Exim and Smail 3 operate on the Sendmail design model, where a single main binary manages all MTA functionalities. Although this monolithic architecture is sometimes viewed as less secure and slower, Exim boasts a superior security record compared to Sendmail and matches the performance of Qmail and Postfix, particularly in advanced functions like queue management, address routing, and testing. Notably, Exim lacks a built-in Graphical User Interface (GUI) for configuration, although some Linux distributions may provide one, enhancing its accessibility for users who prefer a visual approach. As a result, Exim remains a versatile choice for those looking to manage email systems effectively.

Postfix is a mail server developed by Wietse Venema, originally created at IBM research as a viable alternative to the popular Sendmail program, and Wietse continues to enhance and maintain it while working at Google. This robust mail server operates on various UNIX systems including AIX, BSD, HP-UX, IRIX, LINUX, MacOS X, Solaris, and Tru64 UNIX, requiring ANSI C, a POSIX.1 library, and BSD sockets for its functionality. Designed with speed, ease of administration, and security in mind, Postfix features an exterior reminiscent of Sendmail, yet its internal architecture is entirely distinct. Among its key capabilities are the support for multiple SMTP deliveries over a single TLS-encrypted connection, leveraging the existing tlsproxy(8) and scache(8) services, as well as the ability to utilize MySQL stored procedures. Moreover, Postfix implements a gradual degradation feature, allowing a daemon to log warnings and continue operation despite encountering some service issues, rather than terminating abruptly with a fatal error. Additionally, it can set the execute bit on a queue file, which is critical, as failure to do so would prevent any mail from being delivered successfully. This combination of features makes Postfix a versatile choice for mail server management.

qmail serves as a robust, secure, and efficient message transfer agent that prioritizes simplicity. Its emphasis on security is not merely an aspiration but a fundamental necessity, given the critical nature of mail delivery for users, which must remain uninterrupted and entirely secure. With a philosophy that ensures a message, once accepted, is never lost, qmail adopts a straight-paper-path approach to guarantee reliability. Additionally, it offers optional support for maildir, a highly reliable mailbox format that prevents corruption during system crashes—unlike traditional mbox files and mh folders. This feature allows users to read their emails safely over NFS, with the added advantage that multiple NFS clients can deliver emails concurrently. Notably, qmail's design is considerably more compact than other Internet MTAs, which often rely on separate mechanisms for forwarding, aliasing, and mailing lists; instead, qmail streamlines this with a singular forwarding system that empowers users to manage their own mailing lists efficiently. Ultimately, qmail stands out not just for its performance but also for its innovative approach to email management.