In Outlook for the Web or the Outlook Client you have the option of Flagging an email. We mostly use that for reminding us that this email is something we need to follow up on. Making a To-Do list is also a great to tool for getting through your day. Microsoft To-Do combines both.
You can go to Microsoft To-Do via a browser: https://todo.microsoft.com/
You can get it on the Windows 10 Store