Exim is a free, GPL-licensed mail transfer agent powering cPanel hosting and Debian servers. See how it compares to Postfix and Sendmail in 2026.
Exim is a free, open-source mail transfer agent (MTA) that runs on Unix-like operating systems, handling the routing, filtering, and delivery of email between servers. It was created in 1995 by Philip Hazel at the University of Cambridge Computing Service as a more flexible successor to the older Smail-3 mailer, and it is distributed under the GNU General Public License (GPL-2.0-or-later).
Rather than being a consumer-facing app, Exim operates behind the scenes: it is the software that shared-hosting control panels, ISPs, and Linux distributions rely on to actually send and receive SMTP mail. It is the default MTA on Debian Linux and is bundled with cPanel/WHM, one of the most widely used hosting control panels in the world.
Exim's configuration is split into access control lists (ACLs), routers, transports, retry rules, rewrite rules, and authenticators, giving administrators granular control over exactly how each message is accepted, routed, and delivered. A built-in string-expansion language lets policy logic be expressed directly in the configuration file, and a content-scanning framework allows easy integration with antivirus and antispam engines such as ClamAV and SpamAssassin.
It supports modern email authentication and encryption standards including TLS, SMTP AUTH, DKIM, DMARC, ARC, and DANE, and offers extensibility through a local_scan() C API and embedded Perl support for custom logic beyond what the configuration language alone can express.
Exim is completely free and has no paid tiers, subscriptions, or licensed editions; the GPL license guarantees the source code and binaries remain free to use, modify, and redistribute indefinitely. There is no vendor entity selling Exim, so there are no official support contracts - organizations that want commercial-grade help typically hire independent consultancies, some staffed by long-time Exim maintainers, rather than paying a central company.
Project infrastructure such as hosting and continuous integration is covered by volunteer sponsor organizations (for example Mythic Beasts, MacStadium, and Schlittermann) rather than by product revenue, which is typical for community-governed open-source infrastructure projects of this kind.
Exim is a free, open-source mail transfer agent (MTA) that routes and delivers email on Unix-like systems. It was created in 1995 by Philip Hazel at the University of Cambridge and is licensed under the GNU General Public License.
Yes. Exim has always been free under the GPL-2.0-or-later license, with no paid editions, subscription tiers, or usage fees. Anyone can download, modify, and redistribute it.
Philip Hazel retired from Cambridge in 2007, and maintenance passed to a volunteer group known as the Exim Maintainers, including contributors such as Heiko Schlittermann and Jeremy Harris, who coordinate development on GitHub and via public mailing lists.
Both are free, GPL-licensed MTAs, but Postfix uses a modular multi-process queue manager that generally handles very large queues more gracefully, while Exim offers a more flexible single-binary configuration language favored by hosting control panels for granular, per-domain policy.
Exim ships as the default mail server in cPanel/WHM, one of the most widely deployed hosting control panels, and is also the default MTA on Debian Linux, which together give it one of the largest installed bases of any MTA on the public internet.
Yes. Notable examples include CVE-2019-10149, a remote-code-execution flaw actively exploited by a Linux worm in 2019, and the '21Nails' cluster of vulnerabilities disclosed in 2021, making prompt patching important for any Exim deployment.