It means that you requested a receipt on one sent message and it has been confirmed. When you request a receipt the receiving user is asked to confirm when they read your private message.
Or it might have been from one of your iTraders. I think iTraders are automatically set to have confirmations...
By default, this feature is turned off for all PMs sent directly, but for iTrader notifications, the confirmation receipt is requested automatically(you can't stop it, as its automatic PM when you leave an iTrader for someone.)