Is your account configured correctly?
To look at the settings for an account, select and from the sub-menu, then select 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.in the Thunderbird menu bar, or click the Application menu button and select
Your email account provider can give you connection information about their servers (probably on their website). Make sure your account settings match these settings.