IncrediMail is a free, fun, powerful and user-friendly e-mail program that lets you add 1000s of e-mail backgrounds, Emoticons, Ecards, Sounds, Animations, and 3D effects to your e-mails. You also get fun animated characters to notify you of incoming e-mail.
Sylpheed is a simple, lightweight but featureful, and easy-to-use e-mail client. Sylpheed provides intuitive user-interface. Sylpheed is also designed for keyboard-oriented operation, so Sylpheed can be widely used from beginners to power users.
Thunderbird gives you IMAP/POP support, a built-in RSS reader, support for HTML mail, powerful quick search, saved search folders, advanced message filtering, message grouping, labels, return receipts, smart address book LDAP address completion, import tools, and the ability to manage multiple e-mail and newsgroup accounts.