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Unable to see On-Prem calendar after migrating to Exchange Online

Hi,

We have started migrating from Exchange 2016 to Exchange online. This migration will take around 2 months to complete, and meanwhile we will be running a Hybrid environment.
After we have migrated the first few users, we have noticed that users with a mailbox in Exchange Online are unable to see the calendar of users with an on prem mailbox. They get the error "Could not be updated".

Exchange online users are able to see the calendar of other exchange online users. The default calendar sharing permission for all users is: My organization > Can view when I'm busy.

Any idea where to look into?

Thanks in advance. 

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DEMAN-BARCELO (MVP) Thierry
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Hi,

you can find here a little guide to investigate :

https://docs.microsoft.com/en-us/exchange/troubleshoot/calendars/troubleshoot-freebusy-issues-in-exchange-hybrid#does-freebusy-work-on-premises

Particularly here :
https://docs.microsoft.com/en-us/exchange/troubleshoot/calendars/troubleshoot-freebusy-issues-in-exchange-hybrid#cloud-user-cant-see-on-premises-users-freebusy

Did you choose to install an hybrid agent or the classical installation?

In the classical configuration, your should verify that Web services (/EWS) are accessible from Internet.
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ASKER

Hi Deman,

Thank you for your reply. We have configured the Hybrid setup with a hybrid agent on a dedicated server. Basically, I have inherited a Exchange online project at 50%. While testing with pilot users, we've noticed the free/busy sharing is not working. The link you provided helped me a lot because the organization relationship on EXO and On-Prem was not configured at all.
Now, i have configured both, but free/busy sharing is still not working. After diving more into is, i found out a federation trust is needed to be able to use Free/Busy sharing. Federation trust is configured on EXO, but i don't see that in ECP On Prem. The On Premise Exchange servers do not have internet connection (only inbound 25,443 and outbound 25) and therefore i am unable to set this up. Is it possible to setup the federation via the dedicated Exchange Hybrid Configuration Server? 
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DEMAN-BARCELO (MVP) Thierry
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ASKER

Hi Deman,

Thank you very much for the information. I have tested this in the test environment and it did work!.

Thanks again!