sieve-filter: 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-filter - from package: dovecot-sieve

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

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