Our company is merging with another organization, but only some users are moving to a new Microsoft 365 tenant. Half of the mailboxes will remain in the old tenant using olddomain.com, while the others will be migrated to a new tenant using newdomain.com. Is it possible to keep olddomain.com pointing to the old tenant and have that tenant automatically route messages for migrated users to their mailboxes in the new tenant? A cross-tenant trust is already configured. We are using Microsoft's native cross-tenant mailbox migration and have assigned the required data migration licenses. The migrated users appear as MailUsers in the old tenant, but they also still have their olddomain.com addresses assigned in the new tenant, which may be related to the mail-flow issue.
1 Answer
Check the old-domain addresses on the migrated users. A custom domain can generally be verified and used by only one Microsoft 365 tenant at a time, and the migrated users normally should not retain olddomain.com as an active address in the destination tenant. Their destination-side address should usually be based on newdomain.com, with the old tenant’s MailUser forwarding to the destination routing address. Keeping the old address assigned in both places can cause address ownership or mail-flow conflicts. The migrated users can still receive mail sent to olddomain.com through the source tenant, but the source tenant must be the tenant that owns and routes that domain.
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