Till now Gmail allows us to Star or add labels to important emails we received in our inbox so that we can view them easily under starred section or by clicking on that label, now Gmail allows you to keep track of sent messages by assigning a label or starring them before sending to recipients and recipients will see you message but not your labels or stars you assigned.While Composing you can add a label or star it using the labels drop-down menu.

label or star a message before sending