Emails bounces are categorised into two types

• A hard/permanent bounce: This indicates that there exists a permanent reason for the email not to get delivered. These are valid bounces, and can be due to the non-existence of the email address, an invalid domain name (DNS lookup failure), or the email provider blacklisting the sender/recipient email address.
• A soft/temporary bounce: This can occur due to various reasons at the sender or recipient level. It can evolve due to a network failure, the recipient mailbox being full (quota-exceeded), the recipient having turned on a vacation reply, the local Message Transfer Agent (MTA) not responding or being badly configured, and a whole lot of other reasons. Such bounces cannot be used to determine the status of a failing recipient, and therefore need to be sorted out effectively from our bounce processing.