The headers currently are:
Date: Tue, 28 Jun 2022 19:48:02 +0000
X-Mailer: PHPMailer 6.1.7 (https://github.com/PHPMailer/PHPMailer)
The MIME-Version is already there. The Content-Type is set to: multipart/alternative
This is the necessary type to allow both html and plain text versions to be sent. The code automatically produces plain text versions of the code for html clients that don't support viewing html, but it should deliver the html version for any email client that supports it.
Email clients that support html will get the header:
Content-Type: text/html; charset=us-ascii
If you want to change the charset, you can do that in the email settings UI from our extension. I've added it for you and you can see the result on line 53:
$Email->CharSet = "ISO-8859-1";
That should create the correct headers for you.