After struggling with some increased SPAM scoring on emails sent, I think the problem was related to LT extension for thunderbird adding a:
<script data-cookieconsent="ignore" src="moz-extension://8225a48f-40fb-495c-bc27-bf22a87dc655/content/executor.js"></script>
to my html source code.
I had to disable extension one by one and performed tests with mail-tester.com
Scripts within html always increase spam scoring so if this is truly caused by the extension is a major reason not to use it.