This problem is real and very dangerous to younger children. Electronic mail has made communication possible at anytime, whether the receiver is on the other end or not. Messages can be composed and sent and the receiver can open the mail at their convenience. Asychronous means that the communicators need not be logged onto the computer at the same time. A message can be sent and read a week later.
MUDs are synchronous because everyone involved is present at the current time.
You cannot save a message and allow someone to read it later. One precautionary measure E-mail has is that messages can be traced back to the sender. This holds people more accountable for the messages they send. With MUDs and MOOs there is no way to track another user down or communicate with them. Once they log off, they are gone.