mail agents

Sendmail - Message transfer agent (MTA). Sendmail C/deb

Postfix - Fast, easy to administer, and secure Sendmail replacement. IPL-1.0 C/deb

Dovecot - IMAP and POP3 server written primarily with security in mind. MIT/LGPL-2.1 C/deb

Exim - Message transfer agent (MTA) developed at the University of Cambridge. GPL-3.0 C/deb

chasquid - SMTP (email) server with a focus on simplicity, security, and ease of operation. Apache-2.0 Go

MailCatcher - Ruby gem that deploys a simply SMTP MTA gateway that accepts all mail and displays in web interface. Useful for debugging or development. MIT Ruby

Cyrus IMAP - Email (IMAP/POP3), contacts and calendar server. BSD-3-Clause-Attribution C

Piler - Feature-rich email archiving solution. GPL-3.0 C

Stalwart JMAP - JMAP and IMAP server designed to be secure, fast, robust and scalable. AGPL-3.0 Rust/Docker

Courier MTA - Fast, scalable, enterprise mail/groupware server providing ESMTP, IMAP, POP3, webmail, mailing list, basic web-based calendaring and scheduling services. GPL-3.0 C/deb

EmailRelay - A small and easy to configure SMTP and POP3 server for Windows and Linux. GPL-3.0 C++

OpenSMTPD - Secure SMTP server implementation from the OpenBSD project. ISC C/deb

Slimta - Mail Transfer Library built on Python. MIT Python

Stalwart SMTP - Modern SMTP server designed with a focus on security, speed, and extensive configurability. AGPL-3.0 Rust

DragonFly - A small MTA for home and office use. Works on Linux and FreeBSD. BSD-3-Clause C

Haraka - High-performance, pluginable SMTP server written in Javascript. MIT Nodejs

OpenTrashmail - Complete trashmail solution that exposes an SMTP server and has a web interface to manage received emails. Works with multiple and wildcard domains and is fully file based (no database needed). Includes RSS feeds and JSON API. Apache-2.0 Python/PHP/Docker