Accessing another Shared Mailbox in different Office 365 tenancy?

Question

My company (ParentCo.org) has bought another smaller subsidiary (Subs1.com), both of them are using Office 365.

My Finance team wants to use and access Finance@Subs1.com shared mailbox while using their own ParentCo\First.LastName AD login.

So how can I login to the Shared Mailbox in separate/different M365 ExO tenancy?

Answer 1

Not really possible short of giving them credentials and license in that tenant.
https://learn.microsoft.com/en-us/microsoft-365/admin/email/about-shared-mailboxes?view=o365-worldwide

User permissions: You need to give users permissions (membership) to use the shared mailbox. Only people inside your organization can use a shared mailbox.

External users: You can't give people outside your business (such as people with a Gmail account) access to your shared mailbox. If you want to do this, consider creating a group for Outlook instead. To learn more, see Create a Microsoft 365 group in the admin center.

The only avail "sharing" and collaboration methods that are supported are listed here:
https://learn.microsoft.com/en-us/office365/enterprise/office-365-inter-tenant-collaboration

Answer 2

Based on my knowledge, it's not supported to create a shared mailbox and use the credential of a different O365 organization. If you want to use the mailbox, you may have to migrate mailboxes to your own ParentCo organization. For your reference: How to migrate mailboxes from one Microsoft 365 or Office 365 organization to another

Answer 3

Hi @EnterpriseArchitect ,

The user needs to configure both the accounts in the same outlook profile using independent credentials. This being the closest thing.

Below is a similar questions answered.
https://learn.microsoft.com/en-us/answers/questions/52023/accessing-another-shared-mailbox-in-different-offi.html

Regards,

Satyajit