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

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

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