Question Mail quota for the domain, not the user?

Kind of yes, because when you size all mailboxes like the total of your subscription, space used by some mailbox cannot be used by another. For example, if the subscription allows 100 GB and one of your mailboxes occupies 55 GB, for all other mailboxes only 45 GB will be available (but the users will see their individual max size at 100 GB still).
 
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