Did you ever try to create email using the command line interface of Thunderbird?

From my point of view, this command line interface is somehow incomplete and not very smooth to use.
So I created a wrapper application with a clearer interface for Thunderbird to use it in an easier way.

Just download the freeware application (for non commercial use) from
https://www.markus-bader.de/MB-CMDMailerForThunderbird/index.php

Is is easy to use:

• just define your mail message text in a separate html file
• call MB-Command Line Mailer for Thunderbird with command line parameters (add multiple receivers, To, CC, BCC, add multiple attachments)  and
  press Send when Thunderbird has opened the message window