OpenSMTPD

OpenSMTPD is a free, secure, and simple open-source SMTP server daemon that enables Unix-like machines to exchange emails with other SMTP-speaking systems.

🚀 Overview

OpenSMTPD is a lightweight mail transfer agent (MTA) originating from the OpenBSD project and ported to various Unix-like platforms including Linux, FreeBSD, macOS, and more. It implements the server-side SMTP protocol as per RFC 5321 with additional standard extensions, providing a secure-by-default alternative to complex legacy MTAs like Sendmail or Postfix.

The project solves the problem of overly complicated and vulnerability-prone email servers by emphasizing simplicity, privilege separation, and sane defaults. It's designed for straightforward configuration and operation, reducing the attack surface while supporting essential email relay, delivery, and submission functions.

Built out of dissatisfaction with existing SMTP implementations, it was developed by OpenBSD contributors like Gilles Chehade and Eric Faurot to offer a reliable, maintainable MTA for production use. It's ideal for self-hosting email infrastructure without bloat.

✨ Key Features

Secure-by-default design with privilege separation, dedicated users (_smtpd, _smtpq), and non-root operation
Simple declarative configuration syntax in smtpd.conf(5) for MTA, MSA, and MDA roles
Flexible table system (table(5)) for aliases, virtual domains/users supporting static, SQL, LDAP, and more
Powerful filtering API (smtpd-filters(7)) with mta-actions for spam filtering, DKIM, etc.
smtpctl(8) utility for real-time monitoring, queue management, and daemon control
Local delivery to system users, relay via authenticated submission or smarthost
Sendmail compatibility (sendmail(8), newaliases(8), makemap(8))
Upcoming: Ongoing enhancements in portable builds and OpenBSD integration

🛠 Tech Stack

Core: C (84.5%), Yacc/Bison for parser, libevent for async I/O, LibreSSL/OpenSSL for crypto
Build: autoconf, automake, libtool, pkg-config
Tables extras: SQLite, PostgreSQL, BerkeleyDB via OpenSMTPD-extras
Platforms: OpenBSD (upstream), Linux, FreeBSD, NetBSD, DragonFlyBSD, macOS
Auth: Built-in SASL support, PAM integration
Hosting: Self-hosted daemon, Docker images available

👥 Who Is This For?

This project is useful for:

Sysadmins seeking a minimal, secure mail server for internal or public use
Developers building or testing email applications on Unix systems
OpenBSD users and portable enthusiasts wanting production-grade SMTP without complexity

📸 Screenshots / Demo

Live demo: N/A (daemon software; test via telnet smtp port 25)
GitHub: https://github.com/OpenSMTPD/OpenSMTPD
Website: https://opensmtpd.org
Man pages: https://www.opensmtpd.org/manual.html
Wiki (build guide): https://github.com/OpenSMTPD/OpenSMTPD/wiki

🔥 Project Status

🟢 Actively building

Last updated: 2025-10-30

❓ FAQ

Is this project free?

Yes — Free and open source software under the ISC license. Freely usable and reusable.

Is the source code available?

Yes — Portable repository at https://github.com/OpenSMTPD/OpenSMTPD; upstream in OpenBSD CVS at https://cvsweb.openbsd.org/cgi-bin/cvsweb/usr.sbin/smtpd/; releases at https://www.opensmtpd.org/archives/

Can I contribute?

Forks, pull requests, issues, and discussions welcome on GitHub. Join the mailing list at http://www.opensmtpd.org/list.html or IRC #opensmtpd on irc.libera.chat for feedback, bugs, or features.