Microsoft has added a feature allowing users to receive message reminders in the Outlook Web App. After Outlook assesses that a user might have forgotten to respond to or follow up on emails, the user might start seeing these messages brought to the top of their inboxes. If applicable, a maximum of one email will be brought to the top of the user's inbox at a time.
To disable the feature log into https://www.office.com with your SU email address and password. Select Outlook from the list of applications on the left. In Outlook choose the settings icon on the top right then choose View All Outlook Settings. In the Layout section you will find Message Reminders. Select either Show or Don't show depending on your preference.