OpenSMTPD

OpenSMTPD is a free SMTP server developed as part of the OpenBSD project, focused on security, simplicity, and code quality. It provides a complete implementation of the SMTP protocol (RFC 5321) with support for queueing, authentication, virtual domains, and TLS encryption. The server is designed to be easy to configure and audit, with a clean configuration syntax and a privilege-separated architecture that minimizes attack surface.

OpenSMTPD runs on Linux, BSD systems, and macOS. It supports standard integrations with local delivery agents like maildir and mbox, and can relay through other MTAs. The project includes tools like smtpctl for queue management and statistics. The codebase is maintained by the OpenBSD project, with regular releases and a focus on security audits. Configuration is done in a single config file, with clear syntax for filters, tables, and listeners.

The server is suitable for mail server administrators who want a straightforward, secure MTA without the complexity of Sendmail or Postfix. OpenSMTPD is available under the ISC license (open source). It is the default MTA in OpenBSD and has been ported to many Linux distributions. The project has a small but active community and continues to receive updates focused on stability and security.