If you have set an align attribute in an image to right or left, Thunderbird will add a 3 pixel margin these images.
HTML Email Knowledge Base
The accessibility for HTML emails is often overseen, however, with a few tweaks can greatly improve a readers experience using assistive technology.
Just like the Outlook email client, you can target the Windows 10 Mail App using conditional comments.
You can target the Mozilla Thunderbird email client by including the CSS class .moz-text-html, which Thunderbird wraps around the contained email.
Using conditional comments you can target different versions of the email outlook client and include your client specific HTML.