Do you use Salesforce's Pardot email platform? Lucy Mazalon of the Drip just posted a simple how-to guide that explains how you can automatically send a follow up email to people who didn't open your previous email message.
Don't do this to be endlessly annoying to somebody who isn't responding, but instead, keep this functionality in mind when executing a re-engagement step as part of your subscriber lifecycle management strategy. For subscribers who haven't engaged with your mail (haven't opened or clicked) in 6-12 months, send them a re-engagement (winback) email, and then you could even use a process like this to potentially send them one more follow up email, one more chance to re-confirm that they're alive and they want to stay subscribed to your email messages. If they don't respond by that point, let them go. They're no longer active subscribers; at this point they're dead weight that will bring your reputation down at ISPs due to lowered engagement metrics.
Click here to read the article over on the Drip's website.