If you have an Exchange Hybrid Configuration setup, you may have some users that use services of Office365 but still have their mailbox on-premise. These users probably have an Office365 license assigned to them. Selecting these users from the Users list in the Office365 Admin Portal reveals the following message when viewing the primary e-mail address for such a user:

Error
The items you're trying to open couldn't be found.

Additionally, the same error is displayed when attempting to view or edit Exchange Online properties for such a user.

The error is actually a "bug" in the Office365 Admin Portal, caused because the user has an Exchange Online license assigned. Because of thi  ...