If you running Hybrid Environment or just trying to share free/busy information between your on-premise Exchange Organization and Office 365 you may experience that free/busy information for some mailboxes which were recently moved to O365 is not available. Please note that problem does not affect free/busy from on-premise to O365, if you cannot get free/busy for on-premise mailboxes – it must be another issue.
In addition you may see error similar to following in Application Event Log of your Hybrid server:
Event 4002, MSExchange Availability Process 4192: ProxyWebRequest FederatedCrossForest from S-0-0-00
-0000000000-0000000000-000000000-0000 to https://server_name.outl
ook.com/EWS/Exchange.asmx/WSSecurity failed. Caller SIDs: WSSecur
ity. The exception returned is Microsoft.Exchange.InfoWorker.Comm
rvices.Protocols.SoapException: An internal server error occurred
. The operation failed.
That issue is caused by Office 365 Autodiscover service which returns incorrect endpoint for calendar sharing for some mailboxes.
In order to fix the issue you need to configure TargetSharingEpr property in Organization relationship in your on-premise organization. It will force connection directly to correct O365 endpoint without using Autodiscover.
Use following command:
Set-OrganizationRelationship -Identity "On Premises to Exchange Online Organization Relationship" -TargetSharingEpr "https://outlook.office365.com/ews/Exchange.asmx"
“On Premises to Exchange Online Organization Relationship” is default name for on premise to O365 relationship. It could have different name if you not using Hybrid Configuration Wizard.
That will fix the issue and all O365 mailboxes free/busy information should be accessible from on-premise.