A bounce occurs when your email went to a specific address, but the mail server for that address sent it back saying it was "undeliverable". There are many reasons for this: an invalid email address, a mail server block, the recipient marking the message as spam or unsubscribing, or even a full inbox.
A soft bounce happens when the recipient’s address is correct, but the email was still returned by the server before it reached the recipient. A full inbox, a server that’s down, or a message that’s too large can all cause a soft bounce.
A hard bounce happens when your email is considered permanently undeliverable because there was a problem with the recipient’s address, such as a typo or a change of domain name.
The system will attempt to resend a deferred email for up to 72 hours from the first attempt. If the deferred email is still undeliverable at this time, the email address is marked as undelivered and placed on the bounce list.