/var/list/.bin/mimesend is in smartlist 3.15-22.
This file is owned by list:list, with mode 0o755.
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 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84  | #! /bin/sh
: &&O='cd .' || exec /bin/sh "$0" $argv:q # we're in a csh, feed myself to sh
$O || exec /bin/sh "$0" "$@"		  # we're in a buggy zsh
#########################################################################
#	mimesend	Sends a MIME encapsulated file			#
#									#
#	Created by S.R. van den Berg, The Netherlands			#
#########################################################################
#$Id: mimesend,v 1.16 1995/03/20 14:50:17 berg Exp $
echo=echo		# /bin/echo
test=test		# /bin/test
rm=rm			# /bin/rm
cat=cat			# /bin/cat
sed=sed			# /bin/sed
dirname=dirname		# /bin/dirname
basename=basename	# /bin/basename
splitmail=splitmail	# /usr/local/bin/splitmail	# MIME metamail package
mimencode=mimencode	# /usr/local/bin/mimencode	# MIME metamail package
formail=formail		# /usr/local/bin/formail
procmail=procmail	# /usr/local/bin/procmail
$test 1 != $# && $echo "Usage: mimesend filename" 1>&2 &&
 exit 64
$test -z "$archivedir" &&
 $echo "mimesend: Certain environment variables need to be set" 1>&2 && exit 64
TMPF=/tmp/ms$$.
trap "$rm -f \$TMPF*; exit 1" 1 2 3 13 15
$rm -f $TMPF*
filename=$1
name="`$basename $filename`"
dir="`$dirname $filename`"
export name dir filename archivedir mimencode formail sed	# for mimencap
( $formail -rt -I"Subject: archive retrieval: $1" \
   -i"From: SmartList <$listreq>" -A"X-Loop: $listaddr" \
 -i"Reply-To: Please.write.a.new.mail.instead.of.replying@FIRST.WORD.archive" \
   -i"Content-ID: <\"$filename\"%$listreq>" \
   -I"Precedence: bulk" -X "" <$tmprequest
  $test ! -z "$wrongaddress" && $echo "$wrongaddress"
  cd $archivedir
  if $test -r "./$filename" -a ! -d "./$filename"
  then
     case "`sed -e 1q <\"./$filename\"`" in
	Content-[Tt]ype:*) $cat "./$filename" ;;
	*) echo "" ; $decode "./$filename" ;;
     esac
  else
     $echo "MIME-Version: 1.0"
     $echo "Content-Type: text/plain"
     $echo ""
     if $test -d "./$filename"
     then
	$echo "$filename is a directory, use 'ls' instead."
     else
	$echo "File $filename is currently not available."
	$echo "N.B. Filenames are case sensitive!"
     fi
  fi
) | $procmail -pm SHELL=$SHELL ./../.bin/mimencap | $splitmail -p $TMPF
set dummy $TMPF*
shift
if $test $# = 1
then
  $SENDMAIL $sendmailOPT $sendmailOPTq -t <$1
else
  for a in "$@"
  do
     $formail -fb -A"X-Loop: $listaddr" -I"Precedence: bulk" \
 -a"Reply-To: Please.write.a.new.mail.instead.of.replying@FIRST.WORD.archive" \
	<"$a" |
      $SENDMAIL $sendmailOPT $sendmailOPTq -t
     $rm -f "$a"
  done
fi
$rm -f $TMPF*
 |