Adding sister company emails

Matthew Norton 0 Reputation points
2026-10-13T20:14:48.3933333+00:00

I have admin rights for @companyA.com email through outlook. Can I add users to sister company's email @companyB.com without having an "@companyB.com" email address?

Outlook | Web | Outlook on the web for business | Email
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  1. Jay1 Tran 1,720 Reputation points Independent Advisor
    2026-10-13T20:42:21.78+00:00

    Hi Matthew,

    Thank you for reaching out.

    If companyA.com and companyB.com are two separate Microsoft 365 tenants, your admin rights in Company A don't extend to Company B. To add or manage users in companyB.com, the two tenants need to be connected using cross-tenant collaboration in Microsoft Entra ID.

    There are two main approaches:

    Option 1: Multi-tenant organization (MTO) in Microsoft 365

    An MTO is created in the Microsoft 365 admin center by a Global Administrator of the owner tenant, who invites the other tenant to join. Once the invited tenant's Global Administrator accepts, Microsoft Entra ID automatically configures a cross-tenant synchronization configuration named MTO_Sync_<TenantID> and sets up the organization relationship in cross-tenant access settings. After that, you can share users across tenants from the admin center under Settings > Org settings > Organization profile > Multitenant collaboration, which uses cross-tenant synchronization under the hood. Synced users appear in the other tenant's global address list and can access resources without invitation emails or consent prompts.

    Option 2: Microsoft Entra cross-tenant synchronization

    If you need to sync different sets of users to different tenants, or scope provisioning using Entra groups, you'd configure cross-tenant synchronization directly in Microsoft Entra ID. When configured, Entra ID automatically provisions and de-provisions B2B users in the target tenant.

    For more information, please refer to:

    I hope the information I shared earlier was somewhat helpful in addressing your question. If you have any further questions or updates, please don’t hesitate to share. I’m always happy to assist further if needed.   

    Wishing you a great day ahead.

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  2. AI answer

    2026-10-13T20:15:00.6+00:00

    Mailbox permissions can only be set up within the current organization. It is not possible to set up mailbox permissions for users in another organization.

    If the goal is to manage mail for @companyB.com, access must be granted within that organization by an administrator there. For Microsoft 365 business, managing aliases for a user also requires an Exchange-based administrative role such as Exchange Administrator.

    If both domains are part of the same Microsoft 365 organization, an admin can add aliases for users by going to Microsoft 365 admin center > Users > Active users > select user > Aliases > Manage username and email. The alias must use a domain available in the drop-down list.

    AI-generated content may be incorrect. Read our transparency notes for more information.

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