Office 365 - Outlook - Flag or Pin a Message
Information and a link to Microsoft Support about how to flag and pin messages within Outlook for the Web.
To remind yourself to follow up on a message, you can add a follow-up flag. A flag gives you a visual cue that you want to return to that message later. In the message list, flagged items display the symbol
Pinning a message moves it to the top of your inbox so it's easy to find later. A pinned message is indicated by asymbol.
See the link from Microsoft Support below about how to use this flagging and pinning of messages feature within Outlook for the web:
*Need more help with Outlook? Then check out the Microsoft Support website for Outlook help & learning.