With the high percentage of people who see emails first from their mobile device it is critical that all email campaigns and autoresponders be mobile responsive in order to display properly on mobile devices. The most definitive way to test this is to send your campaign to a test list and then look at it on your own phone. This however takes time and email credits. As an alternative you can use the built in “Preview” module in the editor.
In the editor click on the preview button.
Next use your cursor to change the width of the popup window. As you shrink it to the appx width of a mobile phone you will see your email content shrink and re-align to the screen size. Most mobile phones are about 300 to 350 pixels wide so that is a fair place to test.