RobinS574 Hello and thank you for reaching out to the Community! If all the contacts with that domain (@optonline.net) are bouncing, there could be something on their end blocking your campaigns from going through. This could have happened due to a change or update on their network. You’ll want to make sure Constant Contact is still Whitelisted as a safe sender for their domain. If you still need assistance in this matter you can also reach out to our team in Deliverability.