smtpprox: 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/smtpprox - from package: smtpprox

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