No clue (I copied an example email from some website). I just thought that maybe that was why it's not responsive in gmail or outlook.
Can you please try to change your img to this?
<img src="https://lh3.googleusercontent.com/xxxxxxx" style="width: 100%;height: auto" alt="Happy Birthday!" />
For starters you have some text specific styling in there, it won't do anything in an image. I also removed the
width="800" since that's being changed in the style. I also removed the class so that the style is inline (and removed the
!important, you should try to never have to use that).
What this should hopefully do is have the image always be 100%. I don't think you can do a media query in email, so it's either always 100% or always 800px (with the former being more responsive)