If you see a left pointing purple coloured arrow then it means you have 'Replied' to that email. If you see a right pointing blue coloured arrow then it means you have 'Forwarded' that email.

How to easily remember which is which ? If you select an email so you can read it in the Message Pane, you will see various buttons in the header area eg: Reply, Forward, etc Note the 'arrow' icon you see associated with 'Reply' and 'Forward' buttons. They look the same shape and point in same direction as the ones you see adjacent to the email in the Thread Pane/Message List.