smtpprox-loopprevent: 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/smtpprox-loopprevent - from package: smtpprox-loopprevent

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