Thunderbird

With Mail open, click on the "Tools" menu, then click on "Account Settings...".

Click "Add Account...".

"Email account" should be selected for you. If not, select it. Click "Next >".

Enter the name you want displayed when you send email in the "Your Name:" box, and the POP email address you created for your WebMail account in the "Email Address:" box. Click "Next >".

Make sure that "POP" is selected. Enter mail.yourdomain.com in the "Incoming Server:" box. (If this is the email first account you are setting up in Thunderbird, you will also be asked for an Outgoing Server. Enter mail.yourdomain.com as the Outgoing Server as well.) Click "Next >".

Enter the POP email address in the "User Name:" box. Click "Next".

Enter a name for this account in the "Account Name:" box. Click "Next >".

Click "Finish".

If you will be using your WebMail POP account as your primary email account, click "Set as Default".

Click "OK" and you're done! You can now send and receive email using your new POP email account.