libesmtp-config: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/libesmtp-config - from package: libesmtp-devel

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/libesmtp-config - from package: libesmtp-dev

You may install the required package with command apt-get install {package}

Error in other languages:
libesmtp-config: bevel nie gevind nie
libesmtp-config: comandă negăsită
libesmtp-config: comando não encontrado
libesmtp-config: commande introuvable
libesmtp-config: command not found
libesmtp-config: käsku ei ole
libesmtp-config: không tìm thấy lệnh
libesmtp-config: komanda nerasta
libesmtp-config: Kommando nicht gefunden.
libesmtp-config: kommandot finns inte
libesmtp-config: komut yok
libesmtp-config: nie znaleziono polecenia
libesmtp-config: níor aimsíodh an t-ordú
libesmtp-config: no se encontró la orden
libesmtp-config: no s'ha trobat l'ordre
libesmtp-config: opdracht niet gevonden
libesmtp-config: parancs nem található
libesmtp-config: perintah tidak ditemukan
libesmtp-config: príkaz nenájdený
libesmtp-config: příkaz nenalezen
libesmtp-config: команда не найдена
libesmtp-config: командата не е открита
libesmtp-config: 命令找不到
libesmtp-config: コマンドが見つかりません