If you send campaign to an invalid email address or a closed email account, the message will ‘bounce’ because it can not be delivered to nonexistent email address. This is so called Hard Bounce. Such subscribers will be marked as bounced in your subscriber lists, and the next time you send a campaign these subscribers will be excluded from receiving your campaign.
However, if a message can not be delivered due to some temporary issue, like recipient’s email server is temporary offline, this is considered as a Soft Bounce and such email address will not be marked as a bounced subscriber in your subscriber list.