Microsoft has introduced a feature called ‘Focused Inbox’ in Outlook online in Office 365. Initially only email sent directly to you is put into the ‘Focused’ inbox; many of us rely on group emails, however (like class-of lists) and they land in the ‘Other’ and aren’t immediately visible.
Microsoft has support information about this change here,
To turn this off and return to the normal inbox behavior, click on the Gear icon next to your name and select "view all outlook settings" near the bottom. Then select "Mail".
In the ‘Layout’ section, click on ‘Focused Inbox’, select ‘Don’t sort messages’ and click on the ‘Save’ icon.
