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 - Elastic Email
2. Server Name - smtp.elasticemail.com
3. Port - 465
4. Connection security - SSL/TLS
5. Authentication method - Normal password
6. User Name - your elasticemail.com 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.
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”.