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

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

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