If you need help in identifying which of your subscriber email addresses will bounce then I can help.
I’ve written in the past about various warnings that Mailchimp provides and actions that it takes when it identifies issues with subscribers imported into lists. The message below however is new to me and was received by a person that received (very) high bounce rates when sending campaigns.
Your account no longer supports list imports or single opt-in subscribers through the API. To ensure that we remain within industry thresholds, we require that all subscribers to your lists come through our double opt-in methods, such as the signup form associated with your list.