From Lunarpages Web Hosting Wiki
Thunderbird is a free, open source, cross-platform e-mail and news client developed by the Mozilla Foundation. The project strategy is modeled after Mozilla Firefox, a project aimed at creating a web browser.
How to Setup and E-mail Account in Thunderbird
The first step to setting up Thunderbird would be to open it up, and then from the File menu go to New and then Account. On the first screen of the Account Wizard, select E-mail account" and then hit the Next button.
Now for Your Name, fill in the name as you wish for it to appear on your outgoing messages. For e-mail address, enter your e-mail address you are setting up. Hit the Next button.
On the next screen, select POP at the top as the incoming server you are using. Then fill out your:
Incoming Mail (POP3, IMAP, HTTP) server: [mail.yourdomain.com]
Outgoing Mail (SMTP) server: [mail.yourdomain.com or your ISP SMTP server settings]
Now hit the Next button again. On the User Names screen, enter your full e-mail address(such as firstname.lastname@example.org), then hit the Next button. Under account name, put in a name for this account, such as "Work" or "Home". Hit the Next button.
Now review your information you have given the Account Wizard and if everything is correct, hit the Finish button.