To send a one-line message to a logged-in user, type the monitor command:
  SEND NAM MESSAGE
where NAM is his programmer name (initials).  For messages longer
than one line, type just SEND NAM and follow instructions.  If you
don't know the programmer name for a user you can try his real name.

To send a message to a particular terminal, type
  SEND TTYn MESSAGE
where n is the terminal line number; to send to a particular job, type
  SEND n MESSAGE
where n is the job number.

Various switches can be used with the SEND command, including:
  /NOMAIL  /YESMAIL  /MAIL  /SUBJECT  /WHERE  /DIST  /CC  /LIST
  /NODIST  /HEADER   /ARPA

For more information, see the MAIL section and the MAIL appendix in the
Monitor Command Manual, SAILON-54.6, online as MONCOM.BH[S,DOC].