SOGo is a very fast and scalable modern collaboration suite (or groupware). It offers calendaring, address book management, and a full-featured Webmail client along with resource sharing and permission handling. It also makes use of documented standards (IMAP, CalDAV, CardDAV, etc.) and thereby provides native connectivity (without plugins) to many clients such as Microsoft Outlook, Apple iCal, the iPhone, Mozilla Lightning, and a plethora of mobile devices via SyncML. It can reuse any existing email and database infrastructure, avoiding long hours of site restructuring.
Sylpheed is a simple, lightweight but featureful, and easy-to-use email client. It has a well-polished and comfortable user interface. It has many features such as multiple accounts, thread display, MIME, powerful filtering, various search interface, spell checking, and junk mail control. It supports common email protocols such as POP3, IMAP4rev1, SMTP, and security features such as GnuPG and SSL/TLSv1. It is also fully internationalized and multilingualized.
The MIME Email message class composes and sends MIME encoded email messages. It features user-definable headers and body parts, support for plain text and HTML body, headers with non-ASCII text, HTML messages with embedded images, file attachments with content type detection, forwarding of messages as attachments, setting the error delivery address with the Return-Path header, and sub-classes for different delivery methods: mail, SMTP, Qmail, Sendmail, and Microsoft IIS or Exchange pickup folder. It also supports sending personalized bulk mail by replacing the message parts that differ for each recipient.
Open-Xchange Server Edition is a multi-tier, small footprint solution that integrates seamlessly into customer’s architectures, enabling smooth start of operation based on services. The Open-Xchange Server Edition supports different Enterprise Linux systems like SUSE Linux Enterprise Server 10, Red Hat Enterprise Linux 5, and Debian Etch. Users are presented with a highly advanced, improved, and intuitive browser based interface that meets all their email, calendaring, tasking, contacts, and document storage requirements. Further integrations like LDAP-support or ADS are possible. It's designed for medium organizations and educational and public administrations that want to have a customizable communication solution.
SquirrelMail is a standards-based Webmail package. It includes built-in pure PHP support for the IMAP and SMTP protocols, and all pages are rendered in pure HTML 4.0 for maximum compatibility across browsers. It has very few requirements, and is very easy to configure and install. It has all the functionality you would want from an email client, including strong MIME support, address books, and folder manipulation.
Mutt is a small but very powerful text-based MIME mail client. It is highly configurable, and is well-suited to the mail power user with advanced features like key bindings, keyboard macros, mail threading, regular expression searches, and a powerful pattern matching language for selecting groups of messages.
Roundcube Webmail is a browser-based, multilingual IMAP client with an application-like user interface. It provides the full functionality you expect from an email client, including MIME support, address book, and folder manipulation. High extensibility is granted by the plugin API and the user interface is fully skinnable using XHTML and CSS 2.
Heirloom mailx (formerly known as "nail") is derived from Berkeley Mail and provides the functionality of the System V and POSIX mailx commands. Additional features include support for MIME, IMAP (including caching and disconnected use), POP3, SMTP, S/MIME, international character sets, maildir folders, message threading, powerful search methods, scoring, and a Bayesian junk mail filter. Mailx can be used as a mail batch language in nearly the same way as it is used interactively. It can thus act as a mailbox filter, can fetch mail from remote accounts, and can send files as attachments.
Claws Mail is a GTK+ based, lightweight, and fast email client. Almost all commands are accessible with the keyboard. It also has many features such as multiple accounts, POP3/SMTP/IMAP/NNTP support, OpenSSL, GnuPG, and LDAP support, a plugin system, spell checking, filtering, Actions, spam and virus filters, an address book, internationalization, and much more.
VM is a mail reader written in Emacs LISP for GNU Emacs and XEmacs. It can retrieve mail from local spool files or remotely via POP and IMAP, display and send MIME messages, thread messages, auto-sort messages into folders, and manipulate "virtual folders" of messages matching certain criteria. In Emacs versions with X support it provides an interface with toolbars, menus, and mouse-based message selection.