Microsoft Tech

Dealing with duplicate mailboxes in Office 365 hybrid mode

At my employer we recently decided to switch over to Office 365. During the migration periode I have set-up our Exchange 2010 server in Office 365 hybrid mode. Also set-up the Azure Active Directory Sync. All existing users being sync both ways, perfect!

But then it happend. New users. All seemed working OK, until we noticed two users were not receiving e-mail from IT. What could be the problem here?