Why are recipients marked with colored symbols?

The symbols indicate a recipient’s status and are either green, red or blue.

green This recipient is active and receives newsletters that are sent to his respective group or segment.
red The recipient is deactivated or has not been activated yet. They are either unsubscribed from your recipient list or has not confirmed his registration via double opt-in (yet).
blue This address created a bounce. You will find further information on bounces here.