Hi, @Andrea Ardemagni @Andrea Ardemagni
Since you don't have an on-premise Exchange infrastructure, you can rename your domain. You have backup your forst in case of problem.
https://woshub.com/rename-active-directory-domain/
Renaming a forest is risky operation. You can avoid it by adding a additionale DNS zone domain.com on local DNS server and create UPN suufix for domain.com (it's alreday done for your case) When the renaming is completed without issue , the synchronisation with Entra ID should works automaticcaly because Entra connect server matchs user objects in Entra ID and AD based on mail ,UPN or msd-consystencyGUID.
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