Outlook on Windows silently scales your email by 1.5×

> Your carefully laid-out 600px email

thats the first mistake right there - dont treat email like a PDF

> Apple Mail turns dates into calendar links. Write “Sale ends May 5” in your campaign and iOS Mail wraps “May 5” in a tappable suggestion that opens a create-event sheet. The override styling is bright blue with an underline, ignoring whatever colors you set.

the lack of self-awareness is quite something to behold

Well, you wanted to send an advertisement, not an email, right? Text has auto wrap.

> the screenshots all showed the same thing: the hero, the buttons, the spacing

Any email I get with a "hero" image and buttons is getting marked as spam.

I would blame the email over Outlook.

Well, that explains a lot.