Different rules apply. In particular, template size.

Even when coding a website you have to think about how different browsers and screen sizes will render your site. The same applies to email – except you have different limits consider. While there is no set limit in length, email clients have a default width limit of 600px. It’s important to keep in mind that email clients will not use the full width of your screen to display your email – the last thing you want is for the email to look completely different than in your preview! (By the way, this rule doesn’t apply to designing with mobile in mind since they represent about half of the email opens!)

If you want your email to be viewed in one clean, straight view that does not require any extra scrolling from left to right, the 600px rule is something to keep in mind.

If you use a professional editor like Cakemail's Email Designer, it will help you manage all this and even let you make a slightly different version for mobile. Learn more here on Using the email designer.

‍