nullmailer-smtpd: 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-smtpd - from package: nullmailer

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

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