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.

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”.

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