Mozilla Thunderbird 3

Open Thunderbird. Select Edit and load the tab, Account Settings.
Click Account Actions then select Add Mail Account.
Enter your name in the From: field as well as your email address, then supply a password for your email account. Click Continue.
Thunderbird3 will be detecting and finalizing the configuration settings at this point. Leave the automatic configuration utility to finish.
After the process is done, click Edit.
Change the User Name to your complete email address.
Choose POP or IMAP for the Incoming Protocol. You may select STARTTLS from the dropdown menu.
Set mail.yourdomainname.com for the Outgoing Server and make sure the SMTP port is at 25. However, you can also choose 2525 for this.
Click on Re-test Configuration. A green light will be displayed if everything has been correctly entered.
Click on Create Account.