mm: 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/mm - from package: multimail

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