sieve: 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/sieve - from package: mailutils

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

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