On your Email Campaigns Dashboard, you have access to several reporting metrics essential to analyzing the performance of each of your email campaigns. You'll notice that for your sent campaigns, Opens & Clicks are clickable links:
From here, select Email didn't open. This works similarly to "Email Clicks" or "Email Opens" but in reverse -- here we'll build a list of everyone who got your last email, but did not click on it.
Remember, the content of this email can stay the same as these subscribers have not opened your previous email yet! The focus here should be the subject line and pre-header text. Are you offering an incentive such as free shipping or a discount code? Be sure to switch up your subject line to include this!