When you create and respond to messages, you may add tags to them, to help you find them later.   These tags are only visible to you and never leave your computer.   By default, there are three tags defined: Urgent, Important and Marked.  You may define as many as you wish.


In the Messages sidebar you may press the Filter button to reach all defined tags.   Also, you may select messages based on their read status.  



The number next to the tag refers to the number of messages that contain that tag.  Pressing the 'x' button next to that number allows you to delete that tag from all messages.


When you create a new tag, a random color is chosen for that tag, in case you prefer another color you may double click the just created tag and it will choose a different random color.