sup-sync-back-maildir: 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/bin/sup-sync-back-maildir - from package: sup-mail

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