nullmailer-send: 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/sbin/nullmailer-send - from package: nullmailer

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

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