Why let users send messages in the first place? Tell me how you can get any more secure than that.
A set of four castors. Like the bottom of a shopping trolley. Yours for $699
Tell your friends! "Each castor costs one hundred and seventy five dollars. It costs four hundred thousand dollars to run this computer... for twelve seconds. Ah ha ha ha ha ha ha!" (https://youtu.be/jHgZh4GV9G0?t=19)