Plugin adds text/plain version to all notifications - no HTML,styles or images. Just text.
Spam filter SpamAssassin adds negative score, if email has only html-version.
-1.105 MIME_HTML_ONLY Message only has text/html MIME parts.
As mentioned by Mailchimp "Message only has text/html MIME parts" is the second most common spam filter trigger. You can add text-version if you're using Mailer app. For the rest messages you can use plugin.Framework >=1.14.2 is required.