pksieve: 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/pksieve - from package: pktools

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

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