You may notice that your emails appear slightly different from user to user. This is because email clients, unlike web browsers, do not follow any particular standard for rendering HTML. Each client renders HTML using its own set of standard, which can sometimes cause differences in how things in your email appear to users.
When you are creating the HTML for your email, Sendloop recommends using features (such as tables or inline CSS) that are recognized by even the oldest of email providers.
You can use third-party tools to test your emails and check their appearance across various email providers. Two popular solutions you can look into include Litmus and Email on Acid.