html-email-template

You need to ignore all usual standards, you need to use tables, inline styles, img tags etc.

There are some important points that should be pay attention to it:

Use HTML Tables for Layout
Add CSS Styles, actually use inline-style and do not use the CSS style declaration in the HTML head tag, as you might when authoring web pages.
Avoid using JavaScript. Most email software will disable it anyway.
Avoid using float in css rules and using flexbox to make it responsive.
It is better to use base64 code jpeg images.
Test, test, test Finally, it is important to always test your emails on a variety of email clients. And be sure to test continuously, as features and support can change at any time!

Optional for test via gmail

You could test your changes via adding extension such as Free HTML Editor for Gmail by cloudHQ