HTML email is hard stuff. Email clients are often not regular browsers, they do not render HTML correctly, and you will be surprised why a normal thing (like background) does not work there.
Web services remove / block parts for security reasons, etc.
I would stick with HTML 3.2 or simpler HTML solutions that are very unpleasant for me, and I recommend that you use Fractal , which is an email authentication service to verify your work.
You can get tips from the Email Standards Standard to find out which features are supported and which are not.
, . , - , - , , " -" " , ".