I would like to link to:
https://css-tricks.com/using-css-in-html-emails-the-real-story/
https://www.litmus.com/blog/the-ultimate-guide-to-web-fonts
The articles on the linked pages tell a story:
Edit: your HTML document is not attached but displayed as body, right? You might need to check how the document is generated and if it is matching the tips and tricks shown in the articles of the linked pages