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
Nino_Bilic
Microsoft
MicrosoftHmmm... 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 mean
We 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.
For the Scenario1 -did you mean you configured another outlook profile for the shared mailbox and use the user with exchange online mailbox or was it added as part of the existing Exchange online Outlook profile -meaning two mailboxes are showing in the left pane of Outlook? Did this produce the same results where F/B is not working?
