qmail-send: 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-send - from package: qmail
/var/lib/qmail/bin/qmail-send - from package: qmail

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

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