Cross tenant mailbox access

Question

Hi

I have an MTO configuration with 2 tenants, lets say contoso.com and northwind.com, and synchronized some users between the 2 tenants.
Now some contoso users must be able to receive and send emails using the northwind.com smtp domain.
So I assgined an exchange online license to a synchronised contoso user in the northwind tenant.

Is that a vaild configuration? Because currently I am unable to login to the northwind.com mailbox of the synchronized user.

Regards
Peter

Answer 1

No, that's not a valid configuration. Exchange does not "understand" MTO at all, as long as it is concerned, a mailbox can only be provisioned for a "member" user, and it doesn't care it said user has a mailbox in any other tenant/location.

Apart from the mailbox provisioning part, you also have to take care of authentication, as cross-tenant federation scenarios are not supported for ExO. In other words, you need to provision a password for the synced user in the "resource" tenant as well. And the user will only be able to access the mailbox when logging in directly with the resource tenant credentials.

So in other words, while you can hack your way around this and eventually end up with a mailbox for the synced user, this is still not cross-tenant access and most definitely not a supported scenario.

Answer 2

Hi @Peter

Thank you for posting your question in the Microsoft Q&A forum.

Based on my research, if these users have mailboxes in source tenant, it's not supported to assign license and enable mailbox at the same time in target tenant. We have to migrate mailboxes from source tenant to target tenant.

You can check these articles for more details about cross-tenant mailbox migration:

Cross-tenant mailbox migration - Microsoft 365 Enterprise | Microsoft Learn

Introduction to Cross-Tenant Mailbox Migrations | Microsoft Community Hub

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