Hello Ben,
Can this be done ?
Yes, you could use Microsoft 365 Business to unify multiple email domains under a new domain. As suggested by Jimmy, you could check out our official article How to migrate mailboxes from one Microsoft 365 or Office 365 organization to another | Microsoft Learn. This article provides step-by-step guidance on how to migrate mailboxes and service settings from one Microsoft 365 or Office 365 organization to another in a business merger scenario. The general steps you need to follow are to add the new domain to Microsoft 365, then set up email addresses in the new domain, and then set up forwarding email from the old domain.
Hope the above information is helpful to you!
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