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.
550 5.7.1 Unable to relay
This issue occurs because centralized mail transport does not work in Cumulative Update 6.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.