Event ID - 6015

Event Id6015
DescriptionCategorizer is NDRing a recipient with address %1:%2 with reason code %3 (%4).
Event InformationAccording to Microsoft:

This error event occurs because all public folder replication that uses Simple Mail Transfer Protocol (SMTP) must originate from an authenticated source or from a server that has been granted Send As permissions.
A common symptom of this error is the generation of a specific non-delivery report (NDR) message. Specifically, you may receive the following NDR message when you try to replicate public folders from a domain that has a Microsoft Exchange Server 5.5 computer or a Microsoft Exchange 2000 Server computer to a domain that has a Microsoft Exchange Server 2003 computer:
General access denied, sender access denied? The sender of the message does not have the privileges necessary to complete delivery.
MSExchangeTransport Event 9013 may also be logged in the Application log. If it is also logged, there is a specific user action required to resolve the error. To view both these events, set diagnostic logging on the Categorizer category of the MSExchangeTransport service to Maximum.
If you do not see MSExchangeTransport Event 9013 logged with Event 6015, use the information provided in other MSExchangeTransport events to try to resolve this error.

If MSExchangeTransport event 9013 is also logged in the Application log, you can add the SkipPublicMDBRestriction registry key to resolve this error. After you add this registry key, the Exchange Server 2003 computer can process system messages and public folder messages from unauthenticated sources.

Caution:This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restore the Registry" Help topic in Regedit.exe or Regedt32.exe.

To resolve this public folder replication issue:
1.Open a Registry Editor, such as Regedit.exe or Regedt32.exe.
Navigate to HKey_Local_Machine\System
Reference LinksCategorizer is NDRing a recipient with address %1:%2 with reason code %3 (%4).

Description of the Microsoft Windows registry

How to troubleshoot public folder replication problems in Exchange 2000 Server and in Exchange Server 2003

How to restrict the users who can send inbound Internet e-mail to another user or to a distribution group in Exchange 2003

Resolve Anonymous Senders Functionality in Microsoft Exchange 2003

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