Occasionally when a user sends email they may receive a Non Delivery Report (NDR) with error code 5.1.1 from the destination email server such as Microsoft Exchange 2000 and 2003 Server. This is an error message that is indicating that the recipient does not exist on the destination email server. And to add more confusion to the situation it is possible that the intended recipient of the first unsuccessful email is able to send email to the original sender and that original sender can successfully reply back to what was the original recipient.

If there are problems with delivering email to a recipient then a Non Delivery Report will be sent back to the sender. The NDR message is a subclass of a larger group of delivery status notifications. The delivery status notifications define three types of status:

1. Success (2.X.X numeric codes)
2. Persistent transient failure (4.X.X numeric codes)
3. Permanent failures (5.X.X numeric codes)

Most NDRs are sent because of failures so those error codes will be of the form 4.X.X or 5.X.X. This implies that the server is able to identify the reason for the failure and so is able to assign an error code which identifies the failure. The NDR which is sent back to the sender of the original email will contain the assigned error code.

Continue reading Troubleshooting Exchange Error 5.1.1