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.

Here is an example of what an Outlook client might see when they receive the 5.1.1 error message:

Your message did not reach some or all of the intended recipients.

Subject: Original Message
Sent: 3/24/2002 11:40 AM

The following recipient(s) could not be reached:
user@domain.com on 3/24/2002 11:40 AM
The e-mail account does not exist at the organization this message was sent to.
Check the e-mail address, or contact the recipient directly to find out the
correct address.
<server.domain.com #5.1.1>

In this example, the server that is reporting the 5.1.1 error code is listed as “server.domain.com”. It is important to note that sometimes the server that is reporting the problem is not the same as the server that is experiencing the problem.

Another example of the error code is a basic message such as: “511 – recipient address has invalid format”.

Here is a list of reasons why the 5.1.1 error may be produced:

1. There is no email account on the destination email server’s site for the recipient’s email address. This may happen if the email account was moved from one administrative group to another administrative group. If the recipient email user then replies to an old email during the transition then an old administrative group style LegDN will be used and the NDR will be sent back to the sender. The same result would also happen if the moved user did not recreate their Outlook profile.
2. Note that if an administrator tries to take a short cut by simply renaming an account in active directory then it will most likely result in NDR messages being created. A workaround would be to put the old legDN in as an x.500 address. This will prevent NDRs from being generated when email is replied to or sent to the old legdn. Note that it is not a Microsoft recommended practice to change the legdn’s.
3. The email message was sent to obsolete personal address book entries. The email account may once have existed there but has since been removed. The account may no longer exist if the end user has left the company and their account has been deactivated or deleted. It is also possible that the email address used is incorrect.
4. The categorizer rejected delivery because you configured your SMTP contact with characters that do not meet the requirements of RFC0821.

To correct this problem an administrator should verify that the recipient’s email address is formatted correctly and then resend the message. Confirm that the receiver’s email server is able to resolve the recipient’s email address.

If the email was sent to a user internal to the organization within the same domain, then it usually means that the recipient’s email address, as shown in either of the “To”, “Cc”, or “Bcc” fields, does not exist in your organization’s email system. If the email was sent to a user email address external to your company then the recipient’s email address should be verified by contacting administrators responsible for the destination email server site.