A computer network-based messaging system controls the content of messages sent by users. A database includes permitted words and replacement text elements corresponding to at least one permitted word. A message filter receives an entered word or phrase input by a user on a messaging device as part of a message. The message filter replaces the entered word in the message with a corresponding replacement text element stored in the database. A message sending part transmits the message including the replacement text element over a data network for viewing by other users only if every entered word in the message is included in the database. The message sending part transmits the message even though one or words contained in the replacement text element are not permitted words in the database. The system may allow deletion of a replacement text element from a message only in its entirety.