"Free/Busy information couldn�t be retrieved because the attendee's Mailbox server is busy" error in a hybrid deployment or a cross-premises environment

You set up an organization relationship to share information between two organizations. This might be in either a hybrid deployment of Microsoft Exchange Online in Office 365 and your on-premises Microsoft Exchange Server environment or two on-premises Exchange Server organizations.

However, you experience issues when you try to use the organization relationship. For example, free/busy lookups don't work. When a user tries to look up free/busy information about another user, the user receives an error message that resembles the following:�Other features that require the organization relationship and that rely on the Exchange Web Services (EWS) may also be affected. These features include MailTips and delivery reports.

This issue occurs if the following conditions are true:�

• You use the TargetSharingEPR parameter of the organization relationship to specify the EWS endpoint.
• The version of Exchange is different in the two organizations. For example, one organization is running Exchange Server 2010 and the other organization is running Exchange Server 2013.�

The Exchange server sends an EWS request that includes the major version of Exchange (either Exchange Server 2013 or Exchange Server 2010) to the other organization. If the version of Exchange is different, the server in the other organization will generate a 500 response to the requestor.

This issue is most likely to occur after a service upgrade from Office 365 pre-upgrade to Office 365. Office 365 uses Exchange Server 2013. If you're running Exchange Server 2010 in the on-premises environment, the versions of Exchange in Office 365 and in your on-premises environment are different. This combination, together with the misconfiguration of having the�TargetSharingEPR parameter set, causes this issue.

To resolve this issue, connect to the organization by using Windows PowerShell, and then set the TargetSharingEPR parameter to Null. This should be performed for all organization relationships.

To set the TargetSharingEPR parameter to Null, follow these steps:

1. Take one of the following actions:
   • In a hybrid deployment, connect to Exchange Online by using Windows PowerShell. For more information, go to the following Microsoft website:�
     Connect Windows PowerShell to the Service
   • In a cross-premises scenario, open Exchange Management Shell. For more information, go to one of the following Microsoft websites:
     • Exchange 2013: Open the Shell
     • Exchange 2010: Open the Shell
2. Check whether the value of the TargetSharingEPR parameter is blank. To do this, run the following command:

   Get-OrganizationRelationship |fl

3. If the value of�the TargetSharingEPR parameter isn't blank, run the following command:

   set-OrganizationRelationship <name of organization relationship> �targetSharingEPR $Null

It's very important to make sure that Exchange Autodiscover is set up correctly for all organizations that use an organization relationship. If Autodiscover isn't working correctly, there's no supported way for you to use organization relationships.

1. Make sure that Autodiscover URLS can be resolved internally and externally for the organization with which you want to share free/busy information.�
2. Make sure that the organization with which you are sharing free/busy information has its external EWS endpoint set up.

Still need help? Go to the Office 365 Community website.