In Loop, instead of using flags to mark emails for later, we use stars.
You can easily 'star' any email so that you can refer back to it at a later date. Emails within your personal inbox, Shared Inboxes or Team inboxes can all be starred.
At the top of all emails, you'll see a number of icons that allow you to take an action. As expected, the star icon is the one to click on to star an email. You'll know and an email is stared when the star appears solid yellow.
To 'unstar' an email simply click on the star icon so it's no longer solid yellow.
All starred items will appear in 'Starred' under Loop Messages.