There are many, many email clients and devices on which an email could be viewed. You need to be sure your email is going to look the same and be readable everywhere (which is more difficult than you might think!).
According to emailclientmarketshare.com, Apple Mail on iPhone has 27% marketshare, Gmail has 17%, and Outlook only 9%. The remaining marketshare is spread very thinly over multiple email clients.
More importantly, those email clients all render code slightly differently – allowing certain things and not allowing others. Furthermore, each email client might be used on various desktop, table, and mobile devices – making clean coding and then testing extremely important.
Two tools you can use to test emails in are Litmus and Email on Acid. With both services, you paste your email code into their program or email it to a special address they provide and your email is tested in the most common email clients and device viewports. You see a preview of what your email looks like in all of them and then adjust your code as necessary. They also offer additional analytics, spam filter testing, and more.