Is your account configured correctly?

To look at the settings for an account, select Tools > Account Settings in the Thunderbird menu bar, or click the Application menu button  and select Options and Account Settings from the sub-menu, then select Server Settings for the account name from the list on the left. The connection to the incoming mail server is configured at the top of the page on the right.

Your email account provider can give you connection information about their servers (probably on their website). Make sure your account settings match these settings.