There are 2 types of bounces, hard and soft.
A hard bounce is and email that couldn’t be delivered for permanent reasons, such as the address doesn’t exist or the server has blocked you. The email is returned to the sender and is completely undeliverable. When this occurs, your email marketing tool will no longer send emails to those email addresses.
A soft bounce is a temporary issue, where the email reaches the recipient’s email server but bounces back undelivered. Soft bounces could be caused by several reasons, such as a full mailbox, an offline email server, or a message file that’s too large.