How to Setup SMTP Authentication in Thunderbird
Open Thunderbird
Click on Tools at the top of the window.
Click on Accounts Settings from the drop down window.
A window similar to the one below should open.

Next, highlight Outgoing Server at the bottom of the list on the left-hand side.
There will be a window similar to the following;

Highlight the email account that you would like to change and click the "Edit..." button.
Another window will open similar to the following;

Under "Security and Authentication, check the box labeled "Use name and password".
Make sure the that your username is in the User Name field.
Click OK to close this window.
Click OK to close the Account Settings window.
Done!