Most likely you need to remove them from the email suppression list on AWS (people sometimes get added to this if they mark an email from AWS as spam or junk at some point) #
Go to AWS, go to Simple Email Service, make sure you change the region to US East 1 N Virginia, click to remove an email address, enter the email and hit save, you’ll get a success message if it was on the suppression list and has now been removed
