A bounce occurs when a campaign (email, newsletter, etc.) is not delivered to a recipient.
What is a hard bounce?
A hard bounce is generated when you try to send an email to an address or domain that no longer exists. If you are consistently generating a hard bounce rate higher than 5%, there may be a serious problem with how you are populating your lists.
What is a soft bounce?
A soft bounce is a temporary error that does not result in the removal of the recipient from the mailing list, but may delay delivery to that person. Your email reaches the server, but is bounced back before it gets to the intended recipient. A soft bounce might occur if a server is temporarily unavailable or if a mailbox is full. After three failed delivery attempts, it is considered a hard bounce.