In a campaign, you can set any touchpoint after the first touchpoint to look as if its a followup to the previous email (so that it will look like a reply email).

Step 1: Click on your"Campaigns" in the upper left-side of your account:

Step 2: Click on your specific campaign:

Step 3: Select the specific touch point you want to look like a follow-up email

Step 4: Change the type of the template to send as a follow-up to previous email

Once you've selected the type, select "Save" and you are done :)