I wanted to start this off with a bit of background as to why emails render differently. You might be thinking that it’s all the same code or some clients don’t accept some codes but what really is behind this. With a normal website and web browser you have you HTML/CSS and JS and the browser turns this directly into something that humans can understand. Browsers all in all have minimal changes between browsers but some code needs special code to work in all browsers. With email clients there is an additional step and this is where an email preprocessor comes into play. A preprocess is an extra step that clients have before the code reaches the emails rendering email. This preprocessor removes any code that will compromise security, privacy or normal email behavior and in some cases add in it’s own native code. Gmail has the most aggressive preprocessor adding in a lot of extra code and Outlook being on the other end where it doesn’t support a lot of code too. This preprocessor is why emails render differently and why we need to try to foresee what is going to be removed before it hits the preprocessor.