extsmail: 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 Debian possible default full paths are:

/usr/bin/extsmail - from package: extsmail

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

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