A bounce message or just "bounce" is an automated message from an email system, informing the sender of a previous message that the message has not been delivered (or some other delivery problem occurred). The original message is said to have "bounced". This feedback may be immediate (some of the causes described here) or, if the sending system can retry, may arrive days later after these retries end. More formal terms for bounce message include "Non-Delivery Report" or "Non-Delivery Receipt" (NDR), [Failed] "Delivery Status Notification" (DSN) message, or a "Non-Delivery Notification" (NDN). Classification Although the SMTP is a mature technology, counting more than thirty years, the architecture is increasingly strained by both normal and unsolicited load. The email systems have been enhanced with reputation systems tied to the actual sender of the email, with the idea of recipient's email servers rejecting email when a forged sender is used in the protocol. Therefore, two types of email bounces have been created: hard bounces and soft bounces. Both of them affect the IP reputation of the sender because the Email Service Providers (ESPs) consider the total bounce rate as a decision factor when directing the email into a user's Inbox. Briefly, the total bounce rate is calculated as the sum of the hard bounce rate and soft bounce rate. Hard bounces Hard bounces are permanent and they score higher in terms of sender's IP damage. Hard bounces occur when the sender's mail server determines that there is a high likelihood that the recipient is unavailable and is likely to remain so. A few of the occasions when hard bounces occur are when the recipient of the email finds himself in one of the following situations: incorrect identifier/incorrect domain (such as a typo in the email address or in the domain) or his server does not accept emails anymore. In this case, removal of the email addresses that bounce back is mandatory. Soft bounces Soft bounces are temporary. A bounced message that experiences a soft bounce may be tried to be redelivered at another time. Soft bounces happen when the recipient of the email has either a full Inbox and therefore no space to store another email, or a limit on the size of the emails that it is allowed to receive. Additional situations in which a soft bounce appears is a block set up on the recipient's email to mark a certain