qmailctl: 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
/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/qmailctl - from package: qmail-run
/var/lib/qmail/bin/qmailctl - from package: qmail-run

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