I designed a responsive email with several elements, each one with a main image.
To prevent images breaks in iphone resolution and higher, I have:
img { max-width: 320px !important; height: auto !important; }
But then, images that are smaller than 320px scale to this width. I fixed that with:
img { max-width: 100% !important; width: auto !important; height: auto !important; }
This works for the main images, but then the business logo resizes to 100% of its parent container.
How I get the best of each solution?
HTML coding for emails is a PAIN
See this chart: https://www.campaignmonitor.com/css/ (you might have to scroll down the page, but it's there)
There is really little to no consistency between email clients as to what they will recognize. I would recommend just coding a simple page that 'squishes' nicely.