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Hi,@JohnNaguib
Thanks for posting your question in the Microsoft Q&A forum.
For so many users, manual export/import would be the least expensive and probably the fastest option.
If you want if someone sends an email to company-a.com, it should still end up in company-b.com's migration mailbox. You can refer to the steps below:
- After migrating the mailboxes, add and verify the domain company-a.com to the company-b.com tenant.
- Configure email forwarding or transport rules to ensure that emails sent to company-a.com addresses are delivered to the corresponding company-b.com mailboxes.
- Test to make sure that emails are correctly being received and that all data has been migrated successfully. Update DNS records for company-a.com to point to the new tenant's email servers.
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