But more so, external style cannot be applied to a subsection of a web page unless they force it into an iframe, which has all sorts of issues of it's own which is why external CSS is usually ignored. Inline CSS is often stripped by the tag strippers who don't want you turning things on or off... and media queries shouldn't even play into it since the layout should be controlled by the page it's being shown inside (for webmail) or the client itself, NOT your mail.

You'll have to use <table> tags—a method long buried by web developers

While it’s not quite completely table-free, I’ve managed to get The Intermittent Newsletter down to a single table—one that’s not even visible to non-Microsoft email clients. Along the way, I made an effort to make The Intermittent Newsletter accessible to more readers.