greylist: 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/greylist - from package: greylistd

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

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