What would a hard bounce refer to on an email sending report?

 A hard bounce refers to an email that permanently bounced back to the sender because the address is invalid. A hard bounce occurs when the prospect’s email address is incorrect, the domain name does not exist, or the sender is suspected as spam and/or has been blocked7. Marketing Cloud Account Engagement marks prospects with hard bounces as undeliverable and prevents any further email sends to them8. A hard bounce is different from a soft bounce, which is a temporary delivery failure due to reasons such as a full mailbox, a server outage, or a message size limit9. Marketing Cloud Account Engagement retries sending emails to prospects with soft bounces until they are delivered or until they reach five soft bounces, after which they are marked as undeliverable