This problem is usually to do with responsive email templates. There's an easy fix:
If you're using a responsive design for your email template, make sure to include the following data-premailer attribute to the style element in the head.
<style type="text/css" data-premailer="ignore">
For compatibility with as many email clients as possible, we inline all styles that are declared in style attributes in the head of your template. When defining conditional styles using media queries for responsive designs you will need to tell the premailer not to inline the styles by adding data-premailer="ignore" to your style attributes.
All regular styles should go into a regular style tag and any media queries should go into the
data-premailer style tag.