By The GNOME Project
For Everybody

Evolution provides integrated mail, addressbook and calendaring functionality to users of the GNOME desktop.

NOTE: Evolution most likely ships with your Linux distribution.

To compile from source, download the latest version of the source code from the GNOME Project's Evolution page.

Thunderbird Windows, Mac, Linux
By Mozilla
For Everybody

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.

For additional information, please refer to: Mozilla Thunderbird: LSU Overview.