To send email using Thunderbird you will have to modify your outgoing server settings in Thunderbird accordingly. Additionally you can modify settings in Thunderbird that will allow you to include an SMTP API header in your emails if you wish to leverage categories or other features of the SMTP API when sending mail via the Thunderbird client.
Enter the following settings:
To incorporate SMTP API headers:
Create a mail message and just like you would add a CC or BCC to your email, click the drop down option that will now allow you to add a header of the type "X-SMTPAPI" in addition to "To".