Opera comes with a mail client and it is fast. I am not sure how well
it fits in with your email needs.
There is also Eudora: https://wiki.mozilla.org/Eudora_OSE
Zimbra: http://www.zimbra.com/products/desktop.html
Spicebird: http://www.spicebird.com/
Kmail in repositories
Balsa: in repositories
Squirrel Mail: in repositories
Claws: in repositories
Seamonkey: in repositories or from http://www.seamonkey-project.org/