When an email cannot be delivered to its intended recipient, it is marked as a bounce. Bounces are classified into two types: 

Soft 

The email was not delivered due to a temporary problem. This could indicate that the recipient's inbox is full, that their email provider is experiencing problems, or that your email was too large. Sometimes it simply indicates that an out-of-office response was received.

Hard 

The email could not be delivered due to a persistent problem. This could be because the email address does not exist, or because your email provider has blocked delivery of your email. If a contact hard bounces, we remove that email address from your list so it is never contacted again - this is done to protect your sender reputation.