It used to be that people looked at emails primarily through desktop and laptop computers with wide screens. So for years, the expectation was that emails should be designed horizontally, and as wide as possible to fit into modern, wide-screen monitors.
But nowadays, more and more people are viewing emails on their smartphones. As of 2015, 50% of emails were read on mobile. That number has likely gone up since then. So design your emails with mobile viewing in mind. What does that mean?
Firstly, it means vertical is king. Design your emails in a single column. It'll look flawless on mobile, and it'll still work well on desktop clients like Outlook and Thunderbird.
Avoid clutter whenever possible. Too many design elements will distract the reader. Keep your email design clean and concise, and use plenty of white-space.
Most importantly, remember that your marketing is meant to serve a specific purpose (increasing brand awareness, driving traffic to the website, getting people into the store, etc.). Which purpose is your email supposed to serve? What do you want your reader to do? Your email should make it easy for your reader to do that.