External Guest User Unable to Add Room Mailbox Calendar Despite Organization Relationship and Limited Details Permissions

Good day Jose, 

An Organization Relationship allows users in different tenants to view calendar availability, but it doesn't provide direct access to open a Room Mailbox calendar as a shared calendar in Outlook on the web. 

Even when a guest account is added to calendar permissions, Exchange Online still treats that account as an external user. As a result, permissions such as Reviewer or Full Details don't grant access to the Room Mailbox calendar. 

Since a Room Mailbox is intended for room reservations rather than shared calendar access, you may consider the following options: 

Option 1: Use Scheduling Assistant (Recommended) 

If the external user only needs to check room availability: 

Have the external user create a new meeting in Outlook. 

Add the Room Mailbox as the location or attendee. 

Open Scheduling Assistant to view the room's availability. 

Select an available time and send the meeting request if a reservation is needed. 

Option 2: Share a dedicated calendar 

If the external user needs ongoing read only access to a calendar: 

Create or use a licensed internal user mailbox. 

Create a separate calendar (new blank calendar) in that mailbox. 

Apply the appropriate sharing policy. 

Send a calendar sharing invitation to the external user. 

Have the external user accept the invitation from their organization account. 

Please note that this dedicated calendar is separate from the actual Room Mailbox calendar.

Reservations made in the Room Mailbox won't automatically appear in the dedicated calendar. An authorized user must update the calendar manually, or the organization must implement a separate approved synchronization solution. 

If the requirement is to view real time Room Mailbox availability, using Scheduling Assistant and adding the Room Mailbox to a meeting request remains the recommended approach. 

I completely understand how these limitations in Exchange Online can feel disruptive. It’s not just about aesthetics; these differences can genuinely affect how smoothly your organization works and navigates the day. 

As a forum user, I hope you understand that I don’t have the ability to modify or escalate product issues directly. However, I strongly recommend submitting your feedback through the Outlook · Community. This is the most effective way to ensure your voice reaches the product team and can be considered for future improvements.   

Once you've submitted your feedback, feel free to share the link here in the comments. I’ll be happy to upvote it to help bring more visibility.  

Additionally, as other users have raised similar concerns and may not be aware of where to share their feedback, I hope this response helps clarify the situation and suggests a potential next step. Highlighting this information can make it more visible to others in the community who may be facing the same issue, making it easier for them to find guidance and contribute their feedback as well. 

If you have any questions or need further support, please don’t hesitate to reach out at any time. 

Thank you again for your time and for raising this important usability concern. 

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