Two M365 tenants after acquisition, mail routes wrong or fails between them
After a merger/acquisition with two M365 tenants you must define connectors and accepted domains cleanly, otherwise each tenant treats the other as an external relay and mail lands in spam or bounce loops.
Try this first
- 1Inventory which domains belong to which tenant, and decide on an end state: one tenant with multiple accepted domains, or longer coexistence.
- 2In Exchange Online create inbound connectors between tenants with TLS verification on sender domain, not just IP.
- 3For coexistence: enable 'shared domain' in both tenants for the overlapping domain and use centralized mail flow so one tenant is the primary MX target.
- 4Test with a mailbox in each tenant: send cross-tenant and check headers (Received, Authentication-Results) for TLS and alignment.
- 5Set an end date for consolidation. Two tenants forever is expensive and painful for users (Teams, OneDrive, identities).
When to bring us in
If you are stuck in dual-tenant config and mail falls between the cracks, we can untangle mail flow and plan consolidation.
See also
- Domain expires tomorrow and nobody saw the emailAn expired domain doesn't transfer instantly. There's a redemption window, but you pay extra.
- Unsure whether to enable auto-renewDisabling auto-renew only makes sense for domains you'll truly drop. For anything live, just keep it on.
- New registrar asks for auth code, can't find itEPP code or transfer code is the password to move a domain from registrar A to B.
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