To send a one-line message to a user which he will receive the next
time he logs in, use the monitor command:
  MAIL PRG MESSAGE
where PRG is his programmer name (initials).  For messages longer
than one line, type just MAIL PRG 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
printed Monitor Command Manual, which is online as MONCOM.BH[S,DOC],
with updates in MONCOM.UPD[S,DOC].

To find out how to read your mail, type HELP RCV or learn how to use the E
editor (which is the preferred mail reading program -- see E.ALS[UP,DOC]).