Sign in with Microsoft
Sign in or create an account.
Select a different account.
You have multiple accounts
Choose the account you want to sign in with.


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.

Need more help?

Want more options?

Explore subscription benefits, browse training courses, learn how to secure your device, and more.

Communities help you ask and answer questions, give feedback, and hear from experts with rich knowledge.

Was this information helpful?

What affected your experience?
By pressing submit, your feedback will be used to improve Microsoft products and services. Your IT admin will be able to collect this data. Privacy Statement.

Thank you for your feedback!