Courier
The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols, such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, webmail, and mailing list services within a single, consistent, framework. Individual components can be enabled or disabled at will. The Courier mail server now implements basic web-based calendaring and scheduling services integrated in the webmail module. Advanced groupware calendaring services will follow soon.
The Courier mail server's source code should compile on most POSIX-based operating systems based on Linux, and BSD-derived kernels. The Courier mail server should also compile on Solaris and AIX, with some help from Sun's or IBM's freeware add-on tools for their respective operating systems.
The Courier mail server evolved out of several related projects, that merged together (more on that later). The Courier mail server implements SMTP extensions for mailing list management.
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Afterlogic Aurora
Groupware system for businesses and providers, backed with mobile apps. Your private cloud combining groupware, file storage and email in a single, unified environment. Clean and modern interface. Can work with your existing mail server or use its own for the complete solution. iOS/Android apps for Email and Files let your users just type their email/password and never deal with mail server settings or blocked IMAP/SMTP ports. Can be integrated with LDAP authentication or LDAP address book, along with OAuth 2.0 authentication and resources, like Google Drive or Dropbox.
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Horde Groupware
Horde Groupware is a free, enterprise ready, browser based collaboration suite. Users can manage and share calendars, contacts, tasks, notes, files, and bookmarks with the standards compliant components from the Horde Project. Horde Groupware bundles the separately available applications Kronolith, Turba, Nag Mnemo, Gollem, and Trean. It can be extended with any of the released Horde applications or the Horde modules that are still in development, like a bookmark manager, or a file manager. Dynamic, basic and mobile interfaces. Public and shared resources (calendars, address books, tasklists and notepads). Translated into 41 languages including right-to-left scripts and full unicode support. Customizable portal screen including applets for weather, quotes, etc. Import and export of groupware data from other applications. User preferences with global default values.
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Mailu
Mailu is a simple yet full-featured mail server as a set of Docker images. It is free software (both as in free beer & as in free speech), open to suggestions and external contributions. The project aims at providing people with an easily setup, easily maintained and full-featured mail server while not shipping proprietary software nor unrelated features often found in popular groupware.
Main features include:
Standard email server, IMAP and IMAP+, SMTP and Submission with autoconfiguration profiles for clients
Advanced email features, aliases, domain aliases, custom routing
Web access, multiple Webmails and administration interface
User features, aliases, auto-reply, auto-forward, fetched accounts
Admin features, global admins, announcements, per-domain delegation, quotas
Security, enforced TLS, DANE, MTA-STS, Letsencrypt!, outgoing DKIM, anti-virus scanner, Snuffleupagus, block malicious attachments
Antispam, auto-learn, greylisting, DMARC& SPF, anti-spoofing
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