mhmail: 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/mh/mhmail - from package: nmh

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

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