Hi Trevor,
Anyone with a post count above 10, can have up to 1500 messages in their box. It is possible that Karl has less than 10 posts. If so, PMs have been stopped for those users.
Please read: this
LINK to find out why.
So Karl, if you have less than 10 posts, post up! If not and your box is full... wow, that's a lot of messages. Time to clean your box.
Eric