On-premises Calendar not Available to Office 365 Users

Aftab Alam
Aftab Alam linkedin | Updated On - April 26, 2023 |

Read time 4 minutes

A hybrid Exchange configuration is highly beneficial for any business. It lets you gain the advantages of various online applications but allows you to have total control over the on-premises environment. It gives a feel of a single organization having both Exchange Server on-premises and Exchange Online (Office 365). The hybrid arrangement can also be considered as a bridge between the two platforms before adopting Exchange Online completely.

Here is the configuration that users will get in the hybrid environment –

  • There will be a single login ID and Password to login to both on-premises and Exchange Online accounts.
  • The email address is the same for both on-premises Exchange and Exchange Online.
  • All the emails sent from the Exchange will be from the Exchange on-premises as it manages all the message transport. The on-premises Exchange functions as the relay for the Exchange Online messages.
  • The calendar’s Free/Busy information is shareable between the Exchange on-premises and Exchange Online mailboxes.
  • Both the platforms, Exchange on-premises and Online use the same URL to connect with their mailboxes.

Couldn’t Open Calendar issue in a Hybrid Exchange Environment

Sometimes in a hybrid environment, when Exchange Online (Office 365) users try to open their on-premises calendar, they may face issues. Here are some errors which users face in such cases:

A connection couldn’t be made with the shared calendar. Remove the calendar and try to add it again or ask the owner to share it again.
Remove the calendar
Furthermore, the users cannot access the information in the Scheduling Assistant for the user’s mailbox, and the user gets the error ‘Could not be updated.’

Reasons Behind the Calendar Inaccessibility issue in a Hybrid Exchange Environment

The cause behind the unavailability of calendar details in Office 365 is that the default Office 365 calendar permissions are set to ‘None.’ When the user makes a query to the scheduling assistant, the scheduling service functions based on the organization relationship rather than the user’s permissions. That’s why when the user makes a query, the scheduling service checks the organization relationship which is attached to the default account and acts on calendar information like Free/Busy, time, subject, location.

How to solve the Calendar not Opening an issue in a Hybrid Exchange Environment?

The solution will work for the hybrid environment because the Free/Busy and calendar sharing features work differently as compared to the situation where both the users are in the same platform (either Exchange or Office 365).

The default Office 365 calendar permissions make sure how the users can see the Free/Busy information. If the default permission is set to either ‘None’ or ‘Controller,’ then the user cannot see the information. Additionally, the user cannot even see the mailbox calendar. It is due to the nature of this permission because ‘None’ and ‘Controller’ both do not provide any Outlook Calendar visibility settings to the user. Here are two steps which you can follow:

  1. To make the calendar entries visible to the user, the mailbox owner or the Office 365 administrator should change the default permission level to either ‘Free/Busy time’ or ‘Free/Busy time, subject, location.’ After that, the user can see the free/busy status in the Scheduling Assistant and also the data in the calendar.
    Office 365 Administrator
    Note: The ‘Free/Busy time, subject, location’ permission is insufficient to allow the user to access the Calendar completely. The mailbox owner should provide the ‘Reviewer’ permission to the user.
  2. The mailbox owner or administrator can grant the required calendar permissions to the user to make the contents of the calendar accessible.
    There is another method that you can use to allow the users to read the calendar across platforms (on-premises & Office 365).

If the administrator wants to provide the user some control over the calendar, then federated calendar sharing is a suitable option. It includes sending an invitation email to the user who requires full access, and they can open the calendar in Outlook.

The administrator will create a sharing policy at both on-premises and Office 365 and mention the specific domain which will be shared with the user. Mention the internal domain in the policy with the appropriate permission. In the policy, the administrator can assign multiple permissions.

Send the invitation by right-clicking on the calendar and clicking the share option. Allow the desired level of access and share the users who will access the calendar.


In a hybrid environment, sometimes Exchange Online users will face issues with calendar access and free/busy information. With proper permission assignment, these issues can be solved. Also, users can try to back up their calendar data for security reasons. This can be done using Kernel Office 365 Backup and Restore.

Q1. How to share calendar availability with external users of Office 365?

Head over to the admin center and navigate to Settings > Org Settings. From there, click on the Services tab & select Calendar. Here, you get the option to grant permissions to external users who have Microsoft 365 or Exchange. Now, you can confidently share your calendar availability with anyone & they can see the right information at the right time.

Q2. Why is the Outlook calendar not showing availability to others?

If you’re having trouble seeing someone’s availability on your Outlook calendar, there could be a few reasons why. 
One common reason is that the attendee you’re trying to schedule with is not a member of your organization. Unfortunately, Outlook is not able to display free/busy information for individuals outside of your organization. 
Also, if you’re creating a meeting in a calendar that isn’t part of your organization, there may be limitations in what information is available to you. 

Q3. How do I list all calendars a user has access to Office 365?

By using the ‘Get-MailboxFolderStatistics’ command, you can easily view all the calendar folders for a specific user. However, to see any shared calendar permissions for each folder, you’ll need to use the ‘Get-MailboxFolderPermission’ command. With these two commands, you’ll be able to compile a complete list of all the calendars that a user has access to, & any permissions they may have for each one. 

Q4. How to fix the issue “why can’t I see a shared calendar in Outlook 365?” quickly?

If there is an Outlook calendar visibility settings issue, there’s a quick fix that can get things back on track. Deleting the person you’re sharing the calendar with & then re-adding them is the simplest solution. This will re-establish the connection between the two calendars & restore any lost sync.