During a merger, acquisition or divestiture, organizations sometimes work from multiple Microsoft 365 tenants for a while, but want to use one email domain externally. That's tricky, because a domain can only be verified in one tenant. Microsoft has been working on sharing an SMTP domain across tenants, and that feature was in preview for a long time; check its current status in the Microsoft 365 message center and roadmap before you plan on it. The classic options below work in any case.
Why is this hard?
A domain such as contoso.com can only be linked to one tenant as a verified domain. Exchange Online only accepts email for domains verified in its own tenant. So a second tenant can't simply receive or send mail with the same domain.
The options
1. Subdomains per tenant
The simplest approach: each tenant gets its own (sub)domain, such as be.contoso.com and nl.contoso.com. There are no conflicts, but email addresses change.
2. One tenant as the entry point, forwarding to the other
The domain stays verified in tenant A. There you set it up as an Internal Relay domain, and you create mail contacts or mail users for tenant B's users, with their address in tenant B as the target address (for example on tenant B's onmicrosoft.com domain). Inbound mail for the shared domain arrives in tenant A and is forwarded.
For outbound mail with the shared domain, tenant B has to send through tenant A or an email gateway via a connector. That takes extra configuration and care for SPF, DKIM and DMARC.
3. A third-party email gateway
A gateway in front of the MX records can decide which tenant the mail goes to based on the recipient, and can rewrite outbound addresses. Useful for larger or long-running scenarios.
4. Migrate to one tenant
Is the situation temporary? Then it's often better to move the mailboxes with a cross-tenant mailbox migration and then move the domain to the target tenant.
Risks and points of attention
- Wrong routing: mail arriving at the wrong tenant or getting stuck in a loop.
- SPF, DKIM and DMARC: every route that sends with the domain must be in your SPF record and sign correctly.
- Complex management: document the setup well and test thoroughly before going to production.
- Address book: users in both tenants don't see each other automatically. Sync contacts or use the multitenant organization features.
Also see share files across multiple tenants and add a domain to Microsoft 365.
Read also
Share and sync files across multiple Microsoft 365 organizations
Add your own domain to Microsoft 365
Enable DKIM in Microsoft 365 for your own domain
Nederlandstalige versie: Hetzelfde SMTP-domein gebruiken in meerdere Microsoft 365-tenants