A considerable portion of our database is taken up due to the massive amount of PMs a number of our users are storing. We have limited all PM boxes to 500 messages. If you currently have over 500 messages you will not be able to receive additional messages until you have less than 500 messages. Either deleting old messages or downloading them in the various formats available within the PM system can achieve this.
As always, if you have any questions, do not hesitate to ask.