How do I set the importance of an email in the Office 365 Web App?

You can let people know the urgency of a message by setting the importance to High, Normal, or Low.

How to set importance in Office 365 Outlook Web Access:

When composing or replying to an email, click the drop down menu. Hover your mouse over “Set importance” and click on the desired level of importance. By default, importance is set to Normal.