There are emails that never get delivered to the recipient and are sent back or in other words bounced back to the sender. Emails can bounce for more than 30 reasons including: the email address is incorrect or has been closed; the recipient’s mailbox is full, the mail server is down, or the system detects spam or offensive content. It shouldn’t be your job to figure out what to do with them. The email marketing software should automatically decide how to handle them. Usually you will have to set up your preferences once and then the software will handle the bounces automatically.