MailMan is one of the most stable, mature, and robust Web-based email systems available. The overall goal of the system is to provide the simplest possible way to add a Web-based email system to any Web site, so ease of installation and maintenance was the first goal in MailMan's design. It has no dependencies and no known incompatibilities with any Web server or mail server. It supports sending and receiving attachments, a single-frame or multi-frame interface, HTML-formatted messages, user options, address books, mail folders, and more.
eud2mbox is a conversion utility to convert Eudora mailboxes (already mostly mbox format) to real mbox-formatted mailboxes, removing all the html/x-flowed and other garbage. It also reads the TOC file and properly inserts both Status: and X-Status: headers so that mailers like mutt/pine/elm can properly tell what messages are read, new, replied, etc.
Evolution is the GNOME mailer, calendar, contact manager, and communications tool. Evolution represents the next step forward in GNOME applications. The tools which make up Evolution will be tightly integrated with one another and act as a seamless personal information-management tool.
Exmh is an X user interface for MH mail. MH provides a set of UNIX commands that manage folders and mail messages. MH has a zillion features as a result of several years of availability. Exmh provides a graphical interface to many of these features, as well as MIME, PGP/GPG support, NNTP, and more.