mddiff: 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/bin/mddiff - from package: syncmaildir

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

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