Users who have a mailbox in the on-premises environment cannot view free/busy information for mailboxes in Exchange Online in the following scenario:
You have a hybrid deployment of Exchange Online and either on-premises Exchange Server 2016 or Exchange Server 2013.
You've set up an intraorganization connector between the Exchange Online organization and the on-premises Exchange organization.
You've set up an organization relationship between the Exchange Online organization and the on-premises Exchange organization.
This issue occurs if the on-premises Exchange server cannot obtain an authentication token by using OAuth. Exchange doesn't try to use the organization relationship if an intraorganization connector exists.
To resolve this issue, follow these steps.
Step 1: Verify the OAuth certificate
Open the Exchange Management Shell.
Identify the certificate for which the authentication configuration is looking. To do this, run the following command:
If no value is returned in the output for CurrentCertificateThumbprint, create a new certificate. To do this, run the following command: