The parental control settings on the Playstation 3 Network for chat does not restrict your child's ability to receive and send text mails to and from other Playstation 3 Network users. Your child's Online ID is publicly viewed by users of the Playstation 3 Network, and as a result, Playstation 3 Network users may contact your child through the Playstation 3 Network using your child's Online ID. You will not receive copies of any text mails or any communications sent to your child by any user. However, as a parent, you can encourage your child to block certain types of text communications from other Playstation 3 Network users on Playstation 3 Network. Your child may block text communications from a specific Playstation 3 Network user by placing him on his block list through the use of the sender's Online ID. Please note that the parent is not able to block users for the child through the current parental control settings, and it is only the child who may block the user through his own sub account.

Source: Sony