Thunderbird

Thunderbird is a relatively recent email package (currently at version 0.8) created by the Mozilla open-source software project. Inspite of its newness, however, Thunderbird comes recommended with many features, such as IMAP/POP support, support for HTML mail, labels, quick search, smart addressbook, return receipts, advanced message filtering, LDAP address completion, import tools, powerful search, and the ability to manage multiple e-mail and newsgroup accounts.

You can obtain more information and download Thunderbird here.