“MJML is a markup language designed to reduce the pain of coding a responsive email.”
And yet it takes simpler code to create a responsive email than a non-responsive one. You don’t have to use a 1990s-style image table grid to tell people about your weekly specials. You don’t have to specify a font size that ends up being unreadable on phone screens. You don’t have to make multiple columns and then figure out how to make sure they reflow on narrower views.