When sending an email, you may receive an email notice that there were communication failures between Salesforce and Amazon, like this:
"...total of 0 emails sent with 2,234 communication failures between Salesforce and Amazon..."
Soapbox Mailer sends email details to Amazon in batches of 10. As such the number of communication failures between Salesforce and Amazon for an issue with the email Campaign will be 10% of the total number of targets.