Symptoms
Consider the following scenario:
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You have a Microsoft Exchange Server 2013 hybrid environment.
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You install Cumulative Update 6 on the on-premises Exchange Server 2013 server.
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You enable centralized mail transport in the hybrid deployment.Transport routing in Exchange 2013 hybrid deployments.
Note Centralized mail transport routes outgoing email messages from the Exchange Online mailboxes through your on-premises Exchange servers. For more information, see -
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
Cause
This issue occurs because centralized mail transport does not work in Cumulative Update 6.
Resolution
To resolve this issue, install Cumulative Update 7 for Exchange Server 2013.
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.