When we send out EDMs to our contacts, some contacts are blacklisted because the EDM you sent them have hard-bounced or soft-bounced.
hard_bounced - A hard-bounced contact means that the email is unreachable - it may be invalid, non-existent, or deleted.
soft_bounced - means the contact's email exists but the EDM was not delivered by its mail service provider. It is possible that the inbox was full.
Publisher does not mark a contact blacklisted if the soft bounces are not consecutive (3 times).