About a year ago, we announced Exchange Server Security Changes for Hybrid Deployments. This change impacted Exchange hybrid customers who host some of their mailboxes on-premises and their on-premis...
Updated Aug 31, 2026
Version 6.0
Blog Post
In most cases, free/busy functionality works after switching to Graph.
The only scenario where it fails is when a shared mailbox has been added (via a security group) and the user opening that mailbox has their own mailbox in Exchange Online.
In other cases—where the shared mailbox is accessed by a user with an on-premises mailbox—it works.
It even works when the user opening the shared mailbox has no mailbox at all.
In OWA and Developer tools i became this error.
"ResponseMessage": { "MessageText": "Error during proxy web request. , inner exception: System.Net.WebException: The remote server returned an error: (403) Forbidden.\u000d\u000a at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult)\u000d\u000a at Microsoft.Exchange.InfoWorker.Common.Availability.Proxy.RestService.EndFindFreeBusyStatus(IAsyncResult asyncResult)\u000d\u000a at Microsoft.Exchange.InfoWorker.Common.Availability.FreeBusyApplication.EndGraphWebRequest(GraphWebRequest proxyWebRequest, QueryList queryList, RestService service, IAsyncResult asyncResult)\u000d\u000a at Microsoft.Exchange.InfoWorker.Common.Availability.GraphWebRequest.EndInvoke(IAsyncResult asyncResult)\u000d\u000a at Microsoft.Exchange.InfoWorker.Common.Availability.AsyncWebRequest.EndInvokeWithErrorHandling()", "ResponseCode": "ErrorProxyRequestProcessingFailed", "MessageXml": "<?xml version=\"1.0\"?>\u000d\u000a<XmlNodeArray xmlns:t=\"http:\/\/schemas.microsoft.com\/exchange\/services\/2006\/types\" xmlns:m=\"http:\/\/schemas.microsoft.com\/exchange\/services\/2006\/messages\" xmlns=\"http:\/\/www.w3.org\/2001\/XMLSchema-instance\">\u000d\u000a <ExceptionType xmlns=\"http:\/\/schemas.microsoft.com\/exchange\/services\/2006\/errors\">GraphWebRequestProcessingException<\/ExceptionType>\u000d\u000a <ExceptionCode xmlns=\"http:\/\/schemas.microsoft.com\/exchange\/services\/2006\/errors\">5016<\/ExceptionCode>\u000d\u000a <ExceptionServerName xmlns=\"http:\/\/schemas.microsoft.com\/exchange\/services\/2006\/errors\">xxxxx<\/ExceptionServerName>\u000d\u000a <ExceptionMessage xmlns=\"http:\/\/schemas.microsoft.com\/exchange\/services\/2006\/errors\">Proxy web request error. , inner exception: The remote server returned an error: (403) Forbidden. LID: 43532<\/ExceptionMessage>\u000d\u000a<\/XmlNodeArray>", "ResponseClass": "Error" }
when i disable the override, it will works in any cases,,
Get-SettingOverride -Identity "EnableRouteThroughMSGraphFeature" | Set-SettingOverride -Parameters "Enabled=false" -Confirm:$false
Microsoft confirmed this issue
Availability (free/busy) fails for delegated mailboxes in Exchange hybrid deployments using Graph API only
https://support.microsoft.com/en-us/servicing/exchange/server/update/2026/5127092
- Nino_Bilic
Microsoft
Hmmm... where is the shared mailbox? I am not certain that I really get the scenario (I see the error but do not fully get who is accessing what and where all the mailboxes are).
Hi Nico,
Nino_Bilic in all cases the shared Mailbox is onpremise
Scenario1
Own user has a mailbox in exchange Online
Own User is member of a security group which has full access to the shared mailbox
I add this shared mailbox as additonal account in outlook and use my credentials to open the shared mailbox
I am inviting an Exchange Online user from the shared mailbox.
Result / Free Busy not working
Scenario2
Own user has a mailbox in exchange onpremise
Own User is member of a security group which has full access to the shared mailbox
I add this shared mailbox as additonal account in outlook and use my credentials to open the shared mailbox
I am inviting an Exchange Online user from the shared mailbox.
Result / Free Busy working
Scenario3
Own user has no mailbox
Own User is member of a security group which has full access to the shared mailbox
I add this shared mailbox as additonal account in outlook and use my credentials to open the shared mailbox
I am inviting an Exchange Online user from the shared mailbox.
Result / Free Busy working
We have two enviroments and it both enviroments its not working.
I hope this will clear it what i meanWe are seeing the same error, for Your Scenario1, but the results do not match your results. In our case, F/B works from Outlook (old) desktop client, but fails in OWA. I am not sure about Your Scenario 2 & 3. I will give them a try.
