Software

Message transfer agent (MTA)

Access GNU Mailman v3 archives. ([Demo](https://lists.mailman3.org/), [Source Code](https://gitlab.com/mailman/hyperkitty))

Reliable and easy-to-use newsletter tool (alternative to Mailchimp and Sendinblue). ([Demo](https://app.keila.io), [Source Code](https://github.com/pentacent/keila))

High performance, self-hosted newsletter and mailing list manager with a modern dashboard. ([Demo](https://demo.listmonk.app/), [Source Code](https://github.com/knadh/listmonk))

Manage electronic mail discussion and e-newsletter lists. ([Source Code](https://gitlab.com/mailman/))

Marketing automation software (email, social and more). ([Source Code](https://github.com/mautic/mautic))

Newsletter and email marketing with advanced management of subscribers, bounces, and plugins. ([Source Code](https://github.com/phpList/phplist3))

Web user interface to access GNU Mailman. ([Source Code](https://gitlab.com/mailman/postorius/))

GPG-enabled mailing list manager with resending-capabilities. ([Source Code](https://0xacab.org/schleuder/schleuder/tree/master))

Mailing list manager. ([Source Code](https://github.com/sympa-community/sympa))

Feed reader for your email accounts. ([Source Code](https://github.com/cypht-org/cypht))

Browser-based IMAP client with an application-like user interface. ([Source Code](https://github.com/roundcube/roundcubemail))

Simple, modern, lightweight & fast web-based email client (fork of RainLoop). ([Demo](https://snappymail.eu/demo/), [Source Code](https://github.com/the-djmaze/snappymail), [Clients](https://snappymail.eu/repository/v2/plugins/))

Another browser-based IMAP client. ([Source Code](https://sourceforge.net/p/squirrelmail/code/HEAD/tree/))

Modern IRCv3 server written in Go, combining the features of an ircd, a services framework, and a bouncer. ([Source Code](https://github.com/ergochat/ergo))

Modular IRC server written in C++ for Linux, BSD, Windows, and macOS. ([Source Code](https://github.com/inspircd/inspircd))

Responsive web IRC client with theming support. ([Demo](https://kiwiirc.com/nextclient/), [Source Code](https://github.com/kiwiirc/kiwiirc))

Portable and lightweight Internet Relay Chat server for small or private networks. ([Source Code](https://github.com/ngircd/ngircd))

Distributed IRC client, meaning that one (or multiple) client(s) can attach to and detach from a central core. ([Source Code](https://github.com/quassel/quassel))

IRC without netsplits. Distributed IRC server, based on RobustSession protocol. ([Source Code](https://github.com/robustirc/robustirc))

Self-hosted web IRC client. ([Demo](https://demo.thelounge.chat/), [Source Code](https://github.com/thelounge/thelounge))

Modular, advanced and highly configurable IRC server written in C for Linux, BSD, Windows, and macOS. ([Source Code](https://github.com/unrealircd/unrealircd))

Fast, light and extensible chat client. ([Source Code](https://github.com/weechat/weechat))

Advanced IRC bouncer. ([Source Code](https://github.com/znc/znc))