Header image for e-Newsletter not responsive in tests

Hi gang.

I'm just testing out and playing around with the e-Newsletter plugin.
I started to build a sample newsletter, choosing the iLetter template that says it's responsive.
I picked an image for the header that was 800 pixels wide, and in the customizer preview it was acting responsive when re-sizing the browser window. But when I sent myself a preview of the newsletter and received it, the header image was definitely NOT responsive (at least within my Mac Mail client), and I had to scroll sideways to see all the contents of the e-mail.

I tested with a smaller image, and it fit within the e-mail (and the other non-header images within were responsive) -- but then the smaller header image itself wasn't full-width as I would have liked. I was hoping the template was truly responsive and I could just use any image for the header any not have to worry about the width or whether it fit properly or not.

So is it actually NOT a fully responsive template like it states, or am I doing something wrong, or is there another issue going on?

I've granted access to my test site, and the sample newsletter has the original larger header image I was using attached. Feel free to send yourself a test preview of it to see what I mean... (Please don't actually send newsletter to users!)