Official comment
We have an in-depth blog article on different methods to re-send an email to those that do not open it at https://ontraport.com/blog/automation/five-ways-to-resend-an-email
The easiest method is probably viewing the message listing in Contacts > Messages. You can add a header item for "Not Opened" and have ONTRAPORT generate the up-to-date Contact Group when you click the number in that column. You can then either send them a broadcast email, subscribe them to a Step Sequence, or Tag them for action later:
Comment actions