Is the email being viewed in a client that supports responsive email? Most desktop email clients do not support responsive emails;  the responsiveness is mainly for mobile devices or the web page version of the email.

Is the template for the email mobile responsive? You can check the template and change it to responsive by reversing the instructions in this document.  

If the template is responsive, are there any elements in the email that prevent the content from being responsive? Specifically, are there any fixed-widths set within the email? This may be applied to an element like an image or table, and would prevent the message from being responsively resized. 

For example, dotted lines in the email indicate a table. 

Looking at the code for this table, a value of "width="600"" being applied to the "table" element. This will cause the table to be 600px wide in any client that has opened the email, so it will not resize to a smaller width.

To fix this issue, the "width" value should be set to a percentage, such as "width="100%"".