qmail-sendmail: 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-sendmail - from package: qmail

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

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