install-mh: 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/mu-mh/install-mh - from package: mailutils-mh
/usr/bin/mh/install-mh - from package: nmh

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

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