@Luke C , From what you have mentioned , I understand that you have used fabrikam.com to setup M365 tenant . This means that you already have fabrikam.com verified as a domain within the tenant that you have.
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Is it even possible to have an on-prem domain synchronize to an AzureAD domain with a totally different name?
Yes , this is very much possible When you create a azure AD instance , it always gets created as [tenantName].onmicrosoft.com which can be visible to you while creation or this can be skipped depending upon how you have signed up . You can see this later in the azure AD portal which gets listed as primary domain or from Microsoft 365 admin portal where you can go to settings > domains and that will help you see the primary domain name for your Azure AD tenant .
This means you can run multiple different domain emails on the same M365 tenant however you will face some issues like for example.
john@Company portal .com and ******@farikam.com both cannot be present within your Azure AD tenant. John
here is a username which if created once will have a domain identity john
@[tenantName].onmicrosoft.com . Now lets say it was synced from on-premise form the fabrikam OU so I believe the UPN for this user will be ******@fabrikam.com automatically . If you try to create a john@Company portal .com it will not allow you to do so .
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In the Active Directory UPN Suffix, I can see contoso.com (on-prem domain) and beside that in the Azure AD Domain column I see Not Added as though it is expecting to find a matching AzureAD domain.
You are exactly correct , you see that because you need to have the domain contoso.com verified in Azure AD . You can do that by following the article how to add and verify custom domain in Azure AD. If contoso.com is some non-routable domain on the internet and only used internally then you can ignore that message and go ahead . In this case you will have to provide a email suffix @fabrikam.com for all your users or [username]@[tenantName].onmicrosoft.com .
You might already have both contoso.com and fabrikam.com as a domain suffix in your local active directory. And the users in the Fabrikam OU may be using an outside email provider with @fabrikam.com email suffix while other users in other OU may be using @Company portal .com as email suffix. Before you could use @Company portal .com email suffix for other users with this Azure AD you would need to verify the contoso.com in this directory as well. If it is already verified in some other Azure AD tenant then you will need to remove the domain from there and add it here. Removing a custom domain from a Azure AD tenant is a project in itself and you need to be careful about it . I would suggest you to go through the MSLearn module on managing domains.
So as you said its doable but its not a great idea to do this. :) . You may find small issues like this here and there in these scenarios where you are effectively trying to share azure Active directory between multiple organizations. Though it can be done if you are ready to handle a few limitations here and there .
I hope this answers your queries. If the post is helpful , please accept this as answer so that it helps other members in the community with similar queries. Please check the linked articles and learning modules. They will help you understand more on adding and removing domains . Should you have any further queries on this , do let me know and I will help you further.
Thank you.
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