Reply back to email on same thread

Haven't found an existing discussion about this so thought I'd ask. We send an email one day after someone creates an account on our academy site, encouraging them to start a course. If they haven't done so 3 days later, we would like to send a "follow up" email like a reply to the previous email reminding them to take that action. Instead of sending a brand new email, is there a way to make it seem like a reply to the thread of the already sent email from Day 1? These emails are all coming from the program manager so our thinking is that it will look a lot more personalized if it looks like it's a reply to the first email instead of a brand new email thread.