Outlook has let you do this for years, you can set various flags on mail items, you can also do this for mail you send so that it will automatically flag for th

Outlook has let you do this for years, you can set various flags on mail items, you can also do this for mail you send so that it will automatically flag for the recipient. The problem of course is that if you allow the sender to specify the importance of an email they will flag things that are important to them,not necessarily important to you.