Thunderbird is what I use. It's a POP3 mail client. You need to already have the mail set up and you tell it the server location and the user name and all that jazz.
The thing about using thunderbird is that it converts from outlook and such, but you'll never be able to transition FROM it. I don't think anything else reads from thunderbird (should you ever switch in the future).
I got into it using Netscape Mail. Then that disappeared, and I didn't want to keep an ancient version of NS around just for the mail, so I switched. It's basically a stand-alone (and more stable) version of NS mail.