nullmailer-inject: 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/nullmailer-inject - from package: nullmailer

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

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