Once your email is scheduled, the only email addresses that will receive your email are the ones that were on the list at the time you scheduled the email.
Today, you scheduled an email to send out next week. The list you selected was your "Newsletter" list. Tomorrow, you add three more customers to your "Newsletter" list. Those three customers will not receive your scheduled email, since they were added to your list after the email was scheduled.
You would need to reschedule your email, and select your "Newsletter" list again to collect the new customers.