/usr/share/bsd-mailx/mail.help is in bsd-mailx 8.1.2-0.20160123cvs-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 | Mail Command Description
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t [message list] type message(s).
more [message list] read message(s), through the $PAGER
n goto and type next message.
e [message list] edit message(s).
f [message list] give head lines of messages.
d [message list] delete message(s).
s [message list] <file> append message(s) to file.
u [message list] undelete message(s).
R [message list] reply to message sender(s).
r [message list] reply to message sender(s) and all recipients.
p [message list] print message list.
pre [message list] make messages go back to /var/mail.
m <recipient list> mail to specific recipient(s).
q quit, saving unresolved messages in mbox.
x quit, do not remove system mailbox.
h print out active message headers.
! shell escape.
| [msglist] command pipe message(s) to shell command.
pi [msglist] command pipe message(s) to shell command.
cd [directory] chdir to directory or home if none given
fi <file> switch to file (%=system inbox, %user=user's
system inbox). + searches in your folder
directory for the file.
set variable[=value] set Mail variable.
A [message list] consists of integers, ranges of same, :status, /subject, or
user names separated by spaces. If omitted, Mail uses the current message.
The pipe command doesn't accept user names or subject message list, since
that might conflict with the command string.
A <recipient list> consists of recipient addresses or aliases separated by
spaces. Aliases are defined in .mailrc in your home directory.
<file> is a full or relative pathname, +folder, % (system inbox), %user
(specified user's system inbox), # (previous file), & (mbox file), or an
expression understood by ${SHELL:-/bin/sh} -c 'echo ...'.
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