mu: 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/mu - from package: maildir-utils

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