Hi @Steven Lord ,
Setup at least 3 different Custom Domains to host emails for multiple user accounts
You need use the account with Microsoft 365 subscription to manage custom domain operations.
Here is an official article on how to manage accepted domains. Hope this can help you!
Manage accepted domains in Exchange Online | Microsoft Learn
Add a domain to Microsoft 365 - Microsoft 365 admin | Microsoft Learn
Regarding the different types of accounts you use, I recommend posting the question on this site for more professional help: Results in Microsoft 365 and Office - Microsoft Community
Thanks for your understanding!
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