How to share organisational calendar to external people

A room calendar shared between separate Microsoft 365 tenants does not update instantly—use meeting invitations to the room mailbox for live booking, and enable that mailbox to process external meeting requests.

Why new events do not appear immediately

For Microsoft 365 users in different tenants, shared-calendar changes synchronize periodically, generally about every three hours; instant synchronization is currently supported only within the same Microsoft 365 tenant or when Outlook.com is the calendar owner. Updating Outlook or switching between desktop and web will not remove this cross-tenant delay.^1^

If external sharing is disabled and the recipient receives an Internet Calendar Subscription (ICS) instead, refresh timing is controlled by the recipient’s calendar service and can be even slower.

Test by creating an event, waiting at least three hours, and then checking Outlook on the web. If it appears after that interval, the behavior is expected rather than a client synchronization fault.

Editor permission for an external user

Normal external calendar sharing is read-only. Exchange Online sharing policies offer external visibility levels ranging from free/busy to full calendar details, but not Editor access.^2^ A work or school account in another organization is still external because it belongs to a different tenant.

To give someone true Editor or delegate access, they need an account represented as a mailbox-enabled user in your organization. Calendar Editor permission permits creating, modifying, and deleting calendar items; a complete delegate relationship also involves delegate flags and meeting-request delivery settings.^3^

Do not raise the room calendar’s Anonymous permission to emulate edit access. Microsoft recommends keeping unauthenticated access at None, because increasing it can expose calendar information more broadly than intended.^3^

Recommended room-booking design

Instead of allowing the partner to edit the room calendar directly:

1. Have the external organizer create a meeting in their own calendar.
2. Add the room mailbox’s SMTP address as a room/resource attendee.
3. Configure the room mailbox to process external meeting requests.
4. Let Exchange accept, decline, or route the request for approval according to the room’s booking policy.

Connect to Exchange Online PowerShell with an account permitted to manage the room mailbox, then run:

$Room = "[Email]"

Get-CalendarProcessing -Identity $Room |
    Format-List AutomateProcessing,ProcessExternalMeetingMessages

Set-CalendarProcessing -Identity $Room `
    -ProcessExternalMeetingMessages $true

External booking requests are not processed by default; ProcessExternalMeetingMessages $true enables the room mailbox to process them.^4^

Afterward, send a new test invitation from the partner organization and verify that:

• The external organizer receives an acceptance or decline.
• The reservation appears on the room calendar.
• A conflicting request is handled according to the room’s booking policy.

This setting permits external senders who know the room address to submit booking requests, so review the room’s booking restrictions before enabling it. If bookings require approval, configure internal users as resource delegates rather than ordinary mailbox delegates.^5^

Configure what the partner can view

If the partner also needs to view the room calendar, configure an external sharing policy with the minimum required detail:

1. Microsoft 365 admin center → Exchange.
2. Exchange admin center → Organization → Individual sharing.
3. Select the policy assigned to the mailbox.
4. Select Manage domains, edit the partner-domain rule, and choose the required calendar information level.
5. Save both the rule and policy.^6^

Remember that this shared view remains periodically synchronized across tenants; the meeting invitation and room mailbox response should be treated as the authoritative booking workflow.

For a long-term partnership between two Microsoft 365 tenants, Microsoft 365 Cross-Tenant Access Policy is the newer model for calendar availability, calendar sharing, and Places room/desk booking. It is still rolling out, so first check the Microsoft 365 Message Center to confirm availability in both tenants before planning migration.^7^

References

1. Calendar sharing in Microsoft 365 | Microsoft Support
2. Sharing policies in Exchange Online
3. Understand calendar and delegate permissions in Exchange Online
4. Room mailbox doesn't process meeting requests routed through a third-party mail service
5. Use Booking Attendant logs to investigate resource booking issues
6. Modify, disable, or remove a sharing policy in Exchange Online
7. Migrate to Microsoft 365 Cross-Tenant Access Policy for sharing Free/Busy, Calendars, and MailTips