Hey there! First, what email address are you sending from? Did we already verify it to send through our servers?
If so, the second thing I would check (since it's an imported list) is that the AR is set up to send to imported subscribers. Here's an article about that: https://www.authoremail.com/documentation/creating-an-autoresponder-sequence/ (all the way on the bottom).
Basically, you just have to pause, update the AR, check the box, and then save and resume the sequence!