How to Share an Exchange Online Shared Mailbox Across Tenants
Question details
An administrator needs to grant cross-tenant access to a shared mailbox in Exchange Online without creating a duplicate user account in the target tenant.
- Product
- Exchange Online
- Device & OS
- not provided
- Scenario
- An administrator wants to provide an external user from a different Microsoft 365 tenant with the ability to read and send emails from a shared mailbox.
- Observed behavior
- Exchange Online prevents administrators from assigning shared mailbox permissions directly to users from an external tenant.
Verify that you have Global Administrator or Exchange Administrator privileges in both Microsoft 365 tenants before attempting to configure mailbox permissions or create new user accounts.
Create a Local Account in the Target Tenant (Required for Sending)
Since Exchange Online does not natively support cross-tenant Send As or Send on Behalf permissions, you must create a licensed account in the mailbox's tenant if the user needs to reply to emails.
Microsoft currently does not offer a tenant trust feature that allows direct assignment of shared mailbox permissions to an external tenant user. To allow an external user to send emails from the shared mailbox, a local account must be provisioned and licensed in the tenant where the shared mailbox resides.
Log in to the Microsoft 365 admin center of the tenant hosting the shared mailbox. Go to Users > Active users > Add a user, and create a new account for the external person.
During the account creation or in the Licenses and apps tab, assign an Exchange Online license to the newly created user so they can access mailbox features.
Navigate to the Exchange admin center > Mailboxes. Select the shared mailbox, click on Delegation, and add the new user under 'Read and manage (Full Access)' and 'Send as'.
Configure Automatic Forwarding (Receive-Only Alternative)
If the external user only needs to monitor incoming emails and does not need to reply from the shared email address, setting up an automatic forwarding rule is the easiest workaround.
Streamline Your Administrative Documentation with WPS Office
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Frequently Asked Questions
Can I use Azure AD B2B guest accounts to grant access to an Exchange shared mailbox?
No. While Azure AD B2B collaboration allows external guest users to access resources like SharePoint and Microsoft Teams, Exchange Online does not currently support granting shared mailbox permissions (such as Full Access or Send As) directly to guest accounts.
Is it possible to share just the calendar of a shared mailbox across tenants?
Yes, calendar sharing is possible through Organization Relationships and Sharing Policies in Exchange Online. However, this only allows the external tenant to view free/busy information or limited calendar details; it does not grant them access to the mailbox emails.
Will Microsoft add cross-tenant shared mailbox access in the future?
Microsoft frequently updates Microsoft 365 capabilities, but as of now, there is no official roadmap feature that permits direct cross-tenant shared mailbox access for sending emails. Administrators must continue using local user provisioning or cross-tenant synchronization tools for now.
