mug: 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/mug - from package: maildir-utils-extra

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

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