To send mail using Thunderbird you need to modify your outgoing server settings in Thunderbird accordingly.
Additionally, you can modify settings in Thunderbird that will allow you to include a 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.

Start by going to: 'Menu' -> 'Options'->'Account Settings'

Enter the following Settings

1. Description - VMA Emailer

2. Server Name - smtp.elasticemail.com

3. Port - 465

4. Connection security - SSL/TLS

5. Authentication method - Normal password

6. User Name - your visualmarketing.com.au username

Incorporating SMTP API headers requires the following

1. Select Tools > Options (Win) or Thunderbird > Preferences (Mac) from the menu in Thunderbird.

2. Open the 'Advanced' category.

3. Go to the 'General' tab. - Click 'Config Editor':

a) Enter mail.compose.other.header under 'Filter':

b) Double-click mail.compose.other.header.

c) Enter X-SMTPAPI

d) Click OK.

e) Close the configuration editor and preferences dialog.

Restart Thunderbird.

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.