When two organizations use Microsoft 365, users often need to see each other’s Outlook calendars for scheduling meetings or tracking availability. Outlook does not allow calendar sharing across different tenants by default. This article explains how to enable B2B cross-tenant calendar sync using Azure Active Directory settings and Exchange Online configuration. You will learn the exact steps to set up organizational relationships and grant calendar access between tenants.
Key Takeaways: Enabling B2B Calendar Sync Across Tenants
- Azure AD Cross-Tenant Access Settings: Configure inbound and outbound trust settings to allow external users to see calendar data.
- Exchange Online Organization Relationship: Create an organization relationship object that defines sharing rules for calendar free/busy information.
- PowerShell Commands for Exchange Online: Use New-OrganizationRelationship and Set-OrganizationRelationship to automate the setup when the GUI is limited.
How B2B Calendar Sync Works Between Microsoft 365 Tenants
Cross-tenant calendar sharing relies on two components. The first is Azure Active Directory cross-tenant access settings, which control authentication trust between the tenants. The second is Exchange Online organization relationships, which define what calendar data can be shared and at what level of detail.
When a user from Tenant A shares their calendar with a user from Tenant B, the sharing request passes through Azure AD to verify the external user’s identity. If the cross-tenant access policy allows it, Exchange Online checks the organization relationship to determine the sharing scope. The scope options are: free/busy with time only, free/busy with subject and location, or full calendar details.
Both tenants must complete configuration. One tenant alone cannot enable cross-tenant calendar sharing. The tenant that hosts the shared calendar must configure an inbound trust, and the tenant that accesses the calendar must configure an outbound trust. Exchange Online organization relationships must be created on both sides as well.
Prerequisites for Cross-Tenant Calendar Sharing
Before you begin, verify the following requirements:
- Both tenants use Microsoft 365 with Exchange Online.
- You have Global Administrator or Exchange Administrator role in both tenants.
- Azure AD Premium P1 or P2 license is assigned to users in both tenants.
- External sharing in Exchange Online is enabled. Check this in Exchange admin center > Organization > Sharing.
- Users have assigned Exchange Online licenses.
Step 1: Configure Azure AD Cross-Tenant Access Settings
- Sign in to Azure Portal as Global Administrator
Open portal.azure.com. Navigate to Azure Active Directory > External Identities > Cross-tenant access settings. - Add the external tenant
Under Organizational settings, click Add organization. Enter the tenant ID or domain name of the external tenant. Select the tenant from the search results. - Configure inbound access for the external tenant
Click the Inbound access tab. Under Trust settings, check the box for Trust user claims from this organization. This allows Exchange Online to accept the external user’s token. Under B2B collaboration, ensure Allow users to sign in is set to On. - Configure outbound access for the external tenant
Click the Outbound access tab. Under B2B collaboration, ensure Allow users to leave is set to On. Under Access control, select Allow all users. - Save the settings
Click Save at the top of the page. Repeat these steps in the external tenant’s Azure AD for the reverse direction.
Step 2: Create Exchange Online Organization Relationships
- Open Exchange admin center
Go to admin.exchange.microsoft.com. Sign in with an Exchange Administrator account. - Navigate to Organization sharing
In the left menu, expand Organization and select Sharing. Click Add to create a new organization relationship. - Enter the external tenant domain
In the New organization relationship wizard, type the domain name of the external tenant, for example contoso.com. Click Next. - Set sharing level
Under Sharing level, choose Calendar free/busy information with time only or Calendar free/busy information with time, subject, and location. Select the level that meets your business needs. Click Next. - Choose users to share with
Under Which users do you want to share with, select All users in the external organization. Click Next. - Review and create
Review the settings. Click Create. The organization relationship appears in the list. - Repeat on the external tenant
Log in to the external tenant’s Exchange admin center. Create an organization relationship that points to your domain. Use the same sharing level.
Step 3: Verify Cross-Tenant Calendar Sharing
- Share a calendar from the host tenant
In Outlook, open Calendar. Right-click the calendar you want to share and select Sharing > Share Calendar. Enter the email address of the external user. Set the permission level to Can view when I’m busy or Can view all details. Click Send. - Accept the sharing invitation in the external tenant
The external user receives an email invitation. They click Open this calendar. The calendar appears in their Outlook calendar list. - Check the calendar view
The external user sees the shared calendar entries according to the permission level. If only free/busy is shared, they see colored blocks without details.
If B2B Calendar Sync Does Not Work
The external user receives an access denied error
Check the Azure AD cross-tenant access settings on both tenants. Ensure inbound and outbound trust settings are configured correctly. Also verify that the external user has a valid Exchange Online license in their own tenant.
The calendar shows no data or says not shared
Confirm the organization relationship exists on both sides. Use the Exchange Online PowerShell command Get-OrganizationRelationship to list relationships. If one is missing, create it again. Also confirm that the sharing level in the organization relationship matches the permission level set in the calendar share.
Users cannot see free/busy information across tenants
External sharing in Exchange Online may be disabled. Go to Exchange admin center > Organization > Sharing. Under External sharing, select Let users share free/busy information with external users. Click Save.
Azure AD Cross-Tenant Access vs Exchange Organization Relationship
| Item | Azure AD Cross-Tenant Access | Exchange Organization Relationship |
|---|---|---|
| Purpose | Controls authentication trust and identity flow between tenants | Defines what calendar data can be shared and the sharing level |
| Configuration location | Azure portal under External Identities | Exchange admin center under Organization sharing |
| Required for | Any cross-tenant data access including calendar, Teams, and SharePoint | Only Exchange Online calendar sharing |
| Sharing scope | Does not set data scope; only trusts the external user identity | Sets free/busy with time only, with subject and location, or full details |
| Administrator role | Global Administrator | Exchange Administrator |
You can now enable Outlook calendar sharing between two Microsoft 365 tenants using B2B synchronization. The key steps are configuring Azure AD cross-tenant access settings and creating Exchange Online organization relationships on both sides. For advanced scenarios with multiple tenants, consider using PowerShell scripts with New-OrganizationRelationship to automate the process across many domains.