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

Various switches can be used with the MAIL command, including:
  /SUBJECT  /WHERE  /DIST  /CC  /APPEND  /QUEUE  /LIST  /NODIST
  /HEADER   /E      /DATE  /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].

To find out how to read your mail, type HELP RCV.