nullmailer-queue: 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-queue - from package: nullmailer

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

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