qmail-smtpd: 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

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

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