/usr/bin/libesmtp-config is in libesmtp-dev 1.0.6-4.3build1.
This file is owned by root:root, 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 85 86 87 | #! /bin/sh
prefix=/usr
exec_prefix=${prefix}
usage()
{
    cat <<EOF
Usage: libesmtp-config [OPTION]
Known values for OPTION are:
  --prefix=DIR		change libesmtp prefix [default $prefix]
  --libs		print library linking information
  --cflags		print pre-processor and compiler flags
  --plugindir		print authentication plugin location
  --help		display this help and exit
  --version		output version information
  --numeric-version	output version information
  --lib-version		output library version
EOF
    exit $1
}
if test $# -eq 0; then
    usage 1
fi
cflags=false
libs=false
while test $# -gt 0; do
    case "$1" in
    -*=*) optarg=`echo "$1" | sed 's/[-_a-zA-Z0-9]*=//'` ;;
    *) optarg= ;;
    esac
    case "$1" in
    --prefix=*)
	prefix=$optarg
	;;
    --prefix)
	echo $prefix
	;;
    --version)
	echo 1.0.6
	exit 0
	;;
    --numeric-version)
	echo 1.0.6 | awk -F '[a-z.]*' '{ print ($1 * 1000 +  $2) * 1000 + $3 }' 
	exit 0
	;;
    --lib-version)
	echo 7:6:1
	exit 0
	;;
    --help)
	usage 0
	;;
    --cflags)
       	echo -I${prefix}/include -pthread
       	;;
    --libs)
       	echo -pthread -L${prefix}/lib/x86_64-linux-gnu -lesmtp 
       	;;
    --plugindir)
       	echo /usr/lib/esmtp
       	;;
    *)
	usage
	exit 1
	;;
    esac
    shift
done
exit 0
 |