To add a contact to the address book you open the address book window and select New Contact. If you receive an email from someone and want to add them to the address book, when viewing the message in the Message Pane or a tab you will see a star to the right of the senders address in the header bar. If the star is filled with color the contact is already in one of your books. If the star is unfilled you can click on the star to add the contact to the address book. Clicking a color filled star will open the contact edit dialog box so you can add to or change that contacts info. '''This is NOT the same star that is to the left of message headers in the Inbox view.''' The address changing to red in the address field of a new message is just a reminder that this address does not exist in one of your address books. It does not prevent you from sending the message to that address. There has been a bug is a few versions lately where addresses that are in the address book still turn red. I am on version 31.6 and it has this bug. I have not updated to version 38 so I do not know if they have fixed this yet.