proxsmtpd: 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/sbin/proxsmtpd - from package: proxsmtp

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