send: 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/send - from package: mailutils-mh
/usr/bin/mh/send - from package: nmh

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

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