Mozilla Thunderbird

Open Thunderbird.
Select Tools > Account Settings... from the menu at the top of the screen.
The Account Settings window will appear on the screen.
Click on Outgoing Server (SMTP) in the white pane on the left side of the window, then click the Edit... button on the right side of the window.
The SMTP Server window should appear.
In the bottom half of the window under Security and Authentication, make sure the checkbox next to Use name and password is checked, and make sure your email username is entered into the box to the right of the words User Name:. Click OK.
The Account Settings will reappear. Click OK to exit.
When you send an email, Thunderbird will ask for your password. Type it in and save it to be used automatically by Thunderbird if you wish.