Lets start by opening Mozilla Thunderbird:
Go to Tools, then click on Account Settings.
At username:
Select "POP" as your incoming server.
Then click Next.
At Incoming User Name:
At the next screen, enter your email account.
At the next screen, it displays all your email settings.
After you have finished, click on Outgoing Settings (STMP).
Make sure these settings are correct. You can get your outgoing server name by contacting your internet service provider.