Select 'Accounts Settings...' from the Tools Menu of Thunderbird.
Enter your email address into the Email Address field.
Select your current SMTP server and click 'Edit...'. If you have no SMTP server set up yet, click 'Add...'.
Change the Server Name setting to your account's SMTP server. You can see what this is by logging into your SMTP2GO control panel. Change the Port setting to 2525. You can also try using Port 25, 8025 or 587.
Set Connection security to 'None' and Authentication method to either 'Password, transmitted insecurely' or 'Encrypted password'.
Enter your SMTP Username into User Name.
If you wish to use a secure connection, set Connection security to 'STARTTLS' and Authentication method to 'Normal'.
