- You have a hybrid deployment of Microsoft Exchange Online in Microsoft Office 365 and of on-premises Microsoft Exchange Server that contains Microsoft Exchange Server 2013 and Microsoft Exchange Server 2007. Or, two organizations are federated, and at least one of them has Exchange Server 2007.
- Exchange Server 2013 has Cumulative Update 5 installed.
- Exchange Server 2013 and Exchange Server 2007 have different external Exchange Web Services (EWS) URL namespaces.
Note Prior to Cumulative Update 5, when an Exchange Online mailbox queries the free/busy information about an Exchange Server 2007 mailbox, Exchange Server 2013 returns the Exchange Server 2013 EWS endpoint and performs the Wssecurity authentication successfully, and then proxies the EWS traffic to the Exchange Server 2007 Client Access servers.