Bounces are undeliverable addresses to an inbox. When an e-mail bounces, it is classified as either a soft or hard bounce.

Soft bounce

A soft bounce indicates a temporary delivery issue, such as the recipient's inbox is full, their server is down, they're on vacation and have set up their email set to away. 

Hard bounce

A hard bounce indicates an email that cannot be delivered. An example of this is when the email address does not exist.