Sup is a console-based email client for people with a lot of email. It supports tagging, very fast full-text search, automatic contact- list management, custom code insertion via a hook system, and more.

nPOPuk is an ultra-small, but comprehensive, e-mail client that runs on a wide range of Windows-based operating systems

The most powerful platform for email productivity in a beautiful native app — on desktop and mobile.

Eudora started in 1988, at the University of Illinois because the internet was a growing and burgeoning place, but email was not really established on the desktop computers that people were using at the time.

Columba is an email client written in Java, featuring a user-friendly graphical interface with wizards and internalionalization support. Its a powerful mail management tool with features to enhance your productivity and communication.

Experimental terminal mail client aiming for configurability and extensibility with sane defaults.

An enterprise application server that integrates information into a single workspace, integrates also a full mail server

Multi-Account Zoho client for Mac.

Mulberry is a powerful, easy to use electronic mail (email) and calendar program that allows you to send, access, and manage email and calendars. Mulberry also helps you manage other related information, such as contact information.

Nylas Mail Lives is a fork of Nylas Mail that fixes many longstanding issues, improves the client experience, and is committed to publish new releases.

GyazMail is a email client software for Mac OS X, based on Cocoa framework, simple, easy-to-use with many features.

Koma-Mail is an email client which is comfortable and has very handy functions. It does not need an installation, so that you can run Koma-Mail from your flash drive but, of course, also on your desktop.