If the email address is not a Gmail/Gmailified account:
This is a limitation with Gmail's scope of support for media queries, which are used in Online Express emails to implement responsive formatting in the email client. 

If the email address is a Gmail/Gmailified account:
We are currently evaluating this issue and will update this article as we have further updates.

For further information regarding media queries with Gmail, please visit https://litmus.com/blog/gmail-to-support-responsive-email-design