Sending message from one terminal user to another user

It seems that y has turned off messages.

In y's terminal, type:

$ mesg
is n

meaning y does not allow others to write to y's terminal. Then you should try:

$ mesg y

Note

This option y in above command is different with y user in your case.

From man mesg:

NAME
       mesg - control write access to your terminal

SYNOPSIS
       mesg [y|n]

DESCRIPTION
       Mesg  controls the access to your terminal by others.  It's typically used to allow or disallow other users to write to your terminal
       (see write(1)).

OPTIONS
       y      Allow write access to your terminal.

       n      Disallow write access to your terminal.

       If no option is given, mesg prints out the current access state of your terminal.

If X is the usr who wants to pop a message on y machine. Fisrt take the terminal id of y machine and both should be on the same host. you can send a message like below.

1. Type who command and see the terminal id of y machine( for example pty/1 is the terminal ID of y)
2. echo "Hello" 1> pty/1(this will display the message on y machine.