qmail-start: 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/sbin/qmail-start - from package: qmail
/var/lib/qmail/bin/qmail-start - from package: qmail

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

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