You need to first access the sent message log via: Reports and results --> View sent message log, then click on the email you wish to follow up on.

On the next page, click on “Browse list of recipients and search on tracking information”. Click on the grey area “Tracking information search options” detailing the types of searches you can perform. You can select “Show recipients who opened the email”, “Show recipients who didn’t open the email” and many more options.

Then “select all” recipients and you can choose to send a follow-up email, SMS or letter.