Simply go to the "Reports" in the menu available at the top of the page and click on the "Regular Issues". Choose the campaign and select "Errors by contacts." This report will provide you with a list of emails that had an error while receiving your campaign, stating where exactly the error occurred.
The errors are classified as "Temporary" or "Permanent". Please note that the system continues to send the emails to contacts with temporary errors.
All contacts with permanent errors no longer receive your campaigns and can be deleted from the system at any time.