RCV is a program which allows you to examine messages sent to you by
the MAIL command, choosing for each message to delete it, print it,
etc.  To use it, type RCV to the monitor, and say "?" whenever you
are asked a question you don't understand.  NOTE: To exit from RCV,
type "E"--not <call>--to avoid losing some of your mail.
For more information, see the RCV section of the MAIL appendix of
the Monitor Command Manual, which is online as MONCOM.BH[S,DOC].

Since mail files are by default in E-format, you may want to use E instead
of RCV to edit your mail file.  Do so by typing
  ET ∂
(from a display terminal; from a non-display, type ET \MAIL<cr>).  From a
display terminal, E is much more useful than RCV for reading mail.