Mozilla Thunderbird is a fully featured, secure and very functional email client and RSS feed reader. It lets you handle mail efficiently and with style, and Mozilla Thunderbird filters away junk mail too.
The Opera email client is a slick and flexible interface that will satisfy almost all your email needs, and Opera integrates RSS feeds in this experience as well. Some may find the message editor lacking a bit in power, and the absence of support for encrypted email is unfortunate.
Windows Live Mail is a comfortable email program that lets you access IMAP, POP and Windows Live Hotmail accounts with verve and ambition to connect with Windows Live Messenger, Windows Live Calendar, your blog and RSS news feeds, too. Windows Live Mail offers solid spam filters, fast search and a few ready-made search folders, but it could do better helping you organize and manage mail.
Pegasus Mail is one of the most powerful, secure and generally best email programs available for Windows, but the interface could need some polishing to make its features more accessible.
IncrediMail is a fun and easy to use email client that adds spice to the messages you send while protecting you from spam, phishing and fraud attempts in a convenient manner. Unfortunately, IncrediMail lacks some productivity features if you have to deal with large amounts of mail.
Mulberry is a powerful and incredibly versatile email client available for all the popular platforms. If there is something Mulberry lacks, it is simplicity.
Foxmail is a nice email client with lots of great features that make email easy and fun. Unfortunately, it doesn't support IMAP accounts and its message editor lacks basic text (re-)formatting tools.
DreamMail manages email and RSS feeds handily and handsomely with stationery, labels and related message search. Unfortunately, international language support is limited, DreamMail's spam filter seems ineffective, and IMAP accounts are not supported.
Alpine is powerful console email program that makes you use email productively with automation aplenty and nary a distraction.
Sylpheed is a surprisingly versatile email client. 'Surprisingly' because it has a friendly, easy to use interface to its many useful features.
InScribe and its free version, i.Scribe, are true little email program gems. They're small, fast, efficient and very usable. Unfortunately, they lack secure messaging, and the IMAP support in Scribe is not perfect.