Office is full Outlook, Outlook Express was given away with earlier versions of windows and was a cut down mail client.
So, you are most likely talking Outlook itself. That is part of Office. My thoughts are that it ought to work on Windows 10 but I'm afraid I can't be of much help. It sounds like a config issue (or just Windows 10 messing about) but without seeing the machine first hand I can't think of anything specific to suggest.
EDIT : Except to say have you considered either using the inbuilt Mail program (basic but functional), a 3rd party program (such as Thunderbird) or upgrading Office to a more recent version?
Edited by ian72 (Mon 19-Oct-15 14:49:18)