Your ESP might be messing up with the code before it’s sent. Some ESP might do that when they see multiple occurrences of the same property (like width or min-width in this demo) and only keep the last one (the one with the calc() function). Since Outlook Web App filters those, there might be no width left, so everything stacks up. Try to inspect the code in your browser and see what’s left. Also try to send the email using Putsmail. Here are screenshots from the render in Outlook Web App and Office 365.