Open Thunderbird, then click on Tools and go down and click Account Settings:
Now you should see the Accounts Settings window (below). From the list of options on the left, select Server Settings.
In the User Name field, make sure that your full email address (i.e., email@example.com, not just example) is entered. Also, be sure that Authentication Method is set to Encrypted Password (in some cases, selecting Password Transmitted Insecurely may also work).
From the left menu panel again, select Outgoing Server (SMTP). It will show you a summary of your settings. Ensure that the user name listed is your full email address. Also check Port number. If it says 80, it needs to be changed. Acceptable Port numbers are 25 (the default) or 587. In some cases, some of these ports won't work for everyone (which is why we have two of them). If you change this number and suddenly you can't send out email, try a different one. To make changes, click Edit. If you tried all outgoing numbers and still can't send, please contact EONI Technical Support.
Click OK to save your changes and finish up.