postgrey: 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/postgrey - from package: postgrey

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

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