mhnftpmail: 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/share/doc/mh-book/examples/mh/bin/mhnftpmail - from package: mh-book

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

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