We are in the middle of hybrid coexistence migration from Exchange 2010 on-premise to Office 365. That means we have ADFS and "Dirsync" (now called Windows Azure AD Sync) running. We are more than halfway through migrating mailbox, so about 60% of our users' mailboxes are in the cloud and the remaining 40% or so are still in on-premise Exchange 2010 databases.
Today we discovered that one of our users has both an on-premise and an Office 365 mailbox linked to his one AD account. That means that if he opens Outlook on a domain-joined computer and goes through the initial configuration, it uses autodiscover to connect him to his on-premise mailbox, but if he logs on to the Office 365 portal, it shows his cloud mailbox.
Even worse, when a user whose mailbox is in the cloud sends him an e-mail, it goes only to his cloud mailbox, and when a user whose mailbox is still on-premise, it goes only to his on-premise mailbox. So he can't see all his mail all in one place.
How can we "merge" his mail data (final destination: Office 365) and make sure his Outlook "autodiscovers" the Office 365 mailbox and all mail is routed to that mailbox?