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Consider the following scenario:

  • You have a Microsoft Exchange Server 2013 hybrid environment.

  • You install Cumulative Update 6 on the on-premises Exchange Server 2013 server.

  • You enable centralized mail transport in the hybrid deployment.

    Note Centralized mail transport routes outgoing email messages from the Exchange Online mailboxes through your on-premises Exchange servers. For more information, see Transport routing in Exchange 2013 hybrid deployments.

  • A user whose mailbox is located in Office 365 sends an email message to an external recipient.

In this scenario, the email message is not delivered successfully, and the user receives a non-delivery report (NDR) that contains the following error message:

550 5.7.1 Unable to relay


This issue occurs because centralized mail transport does not work in Cumulative Update 6.


To resolve this issue, install Cumulative Update 7 for Exchange Server 2013. 


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

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