An easy way to make an email stand out in your inbox is to flag or star it. If you use flags often enough it may be worth your time to learn the keyboard shortcuts. By pressing these keyboard commands your emails will be flagged or starred and easily noticed at a later time.
Apple Mail—press command/shift/L at the same time and a flag will appear or disappear
Gmail—you can always manually press the star icon next to each message. Or to mark an email as starred with a keyboard command you would need to follow these initial steps–Go to the Gear Icon (Settings) in the top right hand corner. Select Settings. Scroll down and select On for Keyboard Shortcuts. Select Save at the bottom of that page. Now press S to mark it as starred. Press S again to undo the star.
Microsoft Outlook-–PC Users should press Ctrl/Shift/G at the same time. Mac users should press command/shift/G.
Yahoo Mail–While viewing a message press L. This action will star that email. Shift and L will undo the star.