Issue with sharing calendar between onpremise and cloud users in hybrid environment EXCH 2019 <> MS365

In a hybrid Exchange environment, sharing calendars between on-premises users and cloud users can be affected by several factors. Here are some points to consider:

1. Sharing Policies: Ensure that the sharing policies are correctly configured to allow sharing between on-premises and cloud users. You may need to set up organization relationships to facilitate this type of sharing.
2. Authentication Relationship: The on-premises Exchange administrator must establish an authentication relationship with the cloud (also known as "federation") for sharing to work across environments. This is crucial for cross-premises calendar sharing.
3. Permissions: Verify that the users have the appropriate permissions set in their sharing policies. If the sharing policy does not allow the level of detail that the user set in the sharing invitation, they may encounter issues. You can check and update the sharing policy using PowerShell or through the Exchange admin center.
4. Updates and Fixes: Ensure that your Exchange 2019 environment is up to date with the latest patches and updates, as fixes for known issues are periodically released by Microsoft.

If you've checked these areas and are still experiencing issues, you may want to consult the Microsoft documentation or community forums for further troubleshooting steps.

References:

• Unable to accept an external sharing invitation by using Outlook in a hybrid environment
• Sharing in Exchange Online

Please go over the steps in the Hybrid free/busy troubleshooter and let us know if you still have issues after exhausting them: https://learn.microsoft.com/en-us/troubleshoot/exchange/calendars/troubleshoot-freebusy-issues-in-exchange-hybrid