By default emails campaigns are mobile responsive which means they change format when viewed on a mobile device.
You are able to set blocks to 'Not Mobile Responsive' if you don't want blocks to stack on top of one another.
To set a block to 'Not Mobile Responsive' hover over the block and click on the block options (spanner) icon.
In the left side panel, change the drop down under 'Mobile Responsive' to No.
When you hover over the block, you will now see a little red mobile icon in the bottom right corner of the block to indicate this block is not mobile responsive.
You are able to view how an email campaign looks on a mobile device by clicking on 'Preview'
Then clicking on the center button to select Mobile view
The main preview area will change to a mobile phone and you can scroll down on the mobile to see how your email campaign will look.