Email systems support a service called Delivery Status Notification or DSN1 for short. This feature allows end users to be notified of successful or failed delivery of email messages. Examples include sending a report when email delivery has been delayed or when an email message has been successfully delivered.
A non-delivery report or NDR is a DSN message sent by the email server (mail transfer agent or MTA for short) that informs the sender that the delivery of the email message failed. While there are various events that can trigger an NDR, the most common cases are when the recipient of the message does not exist or when the destination mailbox is full.
A simple email message is typically made up of a set of headers and at least one body. An example of this can be seen in figure 1. In this example, the email is sent from email@example.com to firstname.lastname@example.org. If the domain name domain2.com does not exist or does not have an email server, then the MTA at “domain1.com” will send an NDR to email@example.com. When the domain name exists and the MTA at domain2.com is accepting email, the behavior is different. In this case, the domain2.com email server should check if the destination mailbox exists and is accepting emails. If this is not the case, then the MTA should reject the email message. However, many mail servers will accept any email and then bounce the email later on if the destination address does not exist.
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