Sorry, I should have clarified. This was created using the Drag and Drop Editor option, not the HTML Editor option.
It'd be hard to say without looking at the actual template.
In the Drag & Drop Editor, when you switch to mobile preview, you have the option to adjust the padding for mobile devices only, if that's breaking the design. It could also be the image width that's breaking it - try setting it to full width on mobile on all images, and go from there.
I just tried creating a blank template with a bunch of different blocks and it's responsive by default. I haven't changed any of the values on the email settings page.
