sieve-connect: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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-connect - from package: sieve-connect

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