How allow access to shared mailbox for external users (users are external members via cross tenant trust)

Question

Hello,

I have created a trust between 2 tenants A and B. Cross-sync is established from tenant A to tenant B. Users of tenant A are available as external members in tenant B.

Next, I added these external users to shared mail for delegated access for both "Read and manage / Full Access" and "Send as".

However, when I try to add this shared email to the user mailbox it fails with below error:

We encountered an error while adding account. Please try again.

Regards, Anish K.

Answer 1

Mailbox access is only possible within the confines of a single tenant. If you need to provide an external user access to messages sent to a given mailbox, forwarding is something to consider. For access to existing messages, you will need to provision the external user as "member", assign him an Exchange Online mailbox and only then grant access to the original mailbox.

Answer 2

Dear @Anish Kumar,

I hope you’re having a good day.

Thank you for providing the details. I understand that you would like users from Tenant A, who are synchronized into Tenant B as external members through Cross-Tenant Synchronization, to access and use a shared mailbox in Tenant B with Full Access and Send As permissions.

Based on my understanding and from several discussions, references, and product-related materials that discuss Multi-Tenant Organizations (MTO), Cross-Tenant Synchronization, and Exchange Online delegation scenarios, this appears to be a limitation of the current Exchange Online functionality rather than an issue with your configuration.

While cross-tenant synchronization can provision users as B2B/member objects and enables collaboration scenarios across services such as Teams and SharePoint, cross-tenant shared mailbox access and mailbox delegation are still commonly requested capabilities and are not currently documented as a fully supported Exchange Online scenario.

In fact, Microsoft has documented customer demand for scenarios such as cross-tenant mailbox delegation, executive assistant access across tenants, and shared mailbox access for B2B users, which indicates that this remains an area of ongoing interest and feedback from enterprise customers.

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To help ensure business communications can continue without disruption, my recommendation for the time would be to provision the users directly in Tenant B with the appropriate Exchange Online licensing and then assign the required shared mailbox permissions there. This is generally the most reliable approach when users need ongoing access to Exchange Online shared mailboxes.

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In addition, this is a capability that has been requested by many organizations operating in multi-tenant Microsoft 365 environments, particularly those using Cross-Tenant Synchronization and Multi-Tenant Organizations (MTO). Based on the available information, cross-tenant shared mailbox access and mailbox delegation remain common customer requests rather than broadly documented Exchange Online capabilities today.

If this functionality is important to your organization, I encourage you to provide direct feedback to the Microsoft product team. There is already a related feedback item here: Shared Mailbox access cross-tenant

You may consider voting for the existing suggestion and adding your own comments explaining your business scenario and requirements. Customer feedback plays an important role in helping Microsoft prioritize future enhancements, and many Microsoft 365 features have evolved over time based on real-world customer input. Hopefully, this scenario will receive further consideration in future Exchange Online development.

I hope the information above helps clarify the behavior you are experiencing.

If this reply helped answer your question or provided useful guidance for the next steps, please consider marking it as the Accepted Answer. This helps other community members who encounter the same situation identify a potential explanation more quickly and makes valuable information easier to find within the Microsoft Q&A community.

Thank you for your time, and I wish you success with your Microsoft 365 deployment. Have a great day!

Warm regards, 

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