There are two main bounce errors, hard bounce and soft bounce
.
1) Soft bounce -> Is a temporary error, the system will try to send the email again.

2) Hard bounce -> It is a permanent error, for instance if the email doesn't exist.
You can find more information on this tutorial:
http://blog.mailrelay.com/en/2012/05/07/how-does-mailrelay-manage-invalid-email-addresses-bounces