https://kb.mailpoet.com/article/180-bounce-management-in-mailpoet-3

Bounce Management in MailPoet

When an email address bounces, it is marked as Bounced in your subscriber list and also added to a suppression list in our sending service. For that reason, just switching it back to Subscribed status in your site’s dashboard would not work since it also needs to be removed from the suppression list in our service before you can send to them again. Don’t hesitate to contact us if you need to remove a valid email address from the bounce list.