Exchange Connection Problems

How to Share a Microsoft 365 Shared Mailbox Across Tenants

Maira Mehtab Sep 22, 2026 869 views

Question details

The user needs to configure a Microsoft 365 shared mailbox located in one tenant so that users from other external tenants can access and collaborate on it.

Product: Microsoft 365 / Exchange Online
Device & OS: not provided
Scenario: Setting up cross-tenant email collaboration and assigning mailbox access to users in different Microsoft 365 environments.
Observed behavior: By default, shared mailboxes are restricted to the tenant they are created in and cannot be natively mapped or made completely transparent to external tenants without specific guest identity configurations.

Before you start

Ensure you have Global Administrator or Exchange Administrator privileges in the host tenant, and verify that your organization's external sharing policies allow inviting B2B guest users.

Solution 1Recommended

Configure Entra B2B Guest Users and Assign Mailbox Permissions

The most reliable method to share a mailbox across tenants is to invite the external users as B2B guests to the host tenant, then grant them explicit mailbox permissions.

A shared mailbox in one Microsoft 365 tenant cannot be made completely transparent across other tenants natively. Cross-tenant collaboration requires creating guest identities in the host directory, assigning explicit permissions, and configuring appropriate Exchange sharing policies.

Invite external users as B2B guests

In the host tenant (Tenant A), log in to the Microsoft Entra admin center. Navigate to Users > All users, and select 'New guest user'. Send invitations to the users in the external tenants (Tenants B and C).

Accept guest invitations

Instruct the external users to check their email and accept the B2B guest invitation so their guest identity is fully provisioned in Tenant A.

Assign mailbox permissions in Exchange Admin Center

Log in to the Exchange Admin Center in Tenant A. Navigate to Recipients > Mailboxes, select the shared mailbox, and go to the 'Delegation' tab. Add the newly provisioned guest users under 'Read and manage (Full Access)' and 'Send as' permissions.

Alternative: Assign permissions via PowerShell

Connect to Exchange Online PowerShell and run the Add-MailboxPermission cmdlet to grant access, for example: Add-MailboxPermission -Identity "shared@tenantA.com" -User "guest_tenantB.com#EXT#@tenantA.onmicrosoft.com" -AccessRights FullAccess.

Alternative Collaboration Methods: If the shared mailbox is primarily being used to store and share files across tenants, consider using Microsoft SharePoint or Microsoft Teams instead, as they are optimized for cross-tenant document collaboration.

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Frequently Asked Questions

Can external users add the shared mailbox directly to their desktop Outlook client?

Natively mapping a cross-tenant shared mailbox into a desktop Outlook profile via auto-mapping is not supported. Guest users typically need to access the shared mailbox using Outlook on the web (OWA) by navigating to the specific host tenant URL or manually adding it via the 'Open another mailbox' feature.

Do I need extra Microsoft 365 licenses for guest users accessing the shared mailbox?

Generally, no. Shared mailboxes under 50GB do not require their own license. B2B guest users are accessing the resource using their guest identity, and as long as they have an appropriate base license in their home tenant, they do not consume an additional license in the host tenant.

Why isn't auto-mapping working for the guest users?

Auto-mapping relies on Active Directory attributes that do not extend to B2B guest accounts across different tenants. Guest users must manually access the shared mailbox rather than expecting it to automatically appear in their folder pane.

Outlook & Email Problems