Send HTML mail with plain text mime/alternative (for anti-spam)

We use Webform Drupal 8 module to handle newsletter subscription on a website. An html formatted email is sent to confirm subscription. Unfortunately, this email is sent with only content-type:text/html, and you know how picky Spamassassin is about this. It's a real deliverability issue. We'd like to be able to send that email with a content-type:multipart/alternative, with 2 parts : one with text/html content-type and one with text/plain. How can we do this ? We installed "mail system" and "mime mail" modules, but we can't figure out how to use them, or even if it's the right solution. Has anyone ever managed to do this?

You can't do this with Drupal core, you need a helper module like Mime Mail.

The Webform module only does the minimum - it sets the content-type header but does not deal with multipart messages, does not deal with embedding images, embedding CSS/JS, re-writing URLs to be absolute, or adding attachments, etc.

The Mime Mail module was specifically written to allow any Drupal module to send multipart/alternative email with the above features.

You say you can't figure out how to use it? There's some really good documentation at https://www.drupal.org/docs/contributed-modules/mime-mail that should help.