There are Harvard studies on this topic. You can start by sending emails anytime in the morning hours OF THE LEADs timezone. Anytime between 8 am to 10 am is fine. Lets say you sent emails at 9 am. And Lets say Jack opened the email at 3pm, and Ryan opened the email at 11 am, the assistant automatically changes the next scheduled time for Jack and Ryan. Jack will get the next email just 15-30 minutes before 3 pm. And Ryan will get the next email 15-30 minutes before 11 am. Remaining people who do not open the email will continue to get it around 9am.

Lets assume your sequence is of 5 emails. For all those who do not open first 3 emails at 9am, assistant will try a different time. Assistant will change the time by 6 hours, and send everyone the 4th email at 3 pm instead. Assistant will do this only for those people who did not open any email.