When an email bounces, the system returns a message to the sender telling them why their email could not be delivered. This string of information is the bounce message. Bounce messages are not always clear or easy to comprehend. For instance, you may get “550 5.1.1 The email account you tried to reach is not existing.” Other names of Bounce Messages include SMTP Bounce Message, SMTP errors, bounce codes, or error codes.
A bounce message is also called a “bounce.” It is an automated message sent from an email system to notify the sender of a previous message about the failed delivery of the message or any other delivery problem that may have occurred. In this message, the original message is said to have “bounced.” A bounce message may come immediately or days later (after which the sending system must have retried delivery and failed).
A bounce message is also called a “Non-Delivery Notification” (NDN), “Delivery Status Notification” (DSN) message, a “Non-Delivery Receipt” (NDR), or a “Non-Delivery Report.”
Having existed for over three decades, the SMTP is obviously a matured technology. However, the increasing unsolicited and normal load is straining the architecture. The email systems now have reputation systems to help them work better. These systems are linked to the actual sender of the email, based on the idea of the recipient’s email servers rejecting email due to the use of an unreal sender in the protocol. This has led to the creation of two forms of email bounces – soft bounces and hard bounces.
Both types of bounces impact the sender’s IP reputation considering the Email Service Providers (ESPs) prioritize the total bounce rate when deciding whether to direct the email into the inbox of a user or not. The total bounce rate can be estimated by adding the soft bounce rate and the hard bounce rate.
Whenever the mail server of the sender detects the high possibility of the recipient being unavailable, it leads to a hard bounce. A hard bounce may occur if the email recipient finds himself in a situation where their server does not accept emails anymore or their domain or identifier is incorrect, perhaps due to a typo in the domain or email address. When this happens, it is compulsory to remove the email addresses that bounce back. Hard bounces are permanent and do more damage to the sender’s IP.
A soft bounce happens when an email recipient has reached the limit of the size of emails they can receive or have no space in their inbox for new emails. A soft bounce may also occur when there is a block in place on the recipient’s email to always mark a specific sender as ‘spam’ or blacklist a specific sender. Likewise, a temporary error from the recipient’s email server or a temporary suspension of their email may lead to a soft bounce. When a bounced message encounters a soft bounce, the delivery may be retried at another time. Therefore, soft bounces are not permanent.