whom: 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

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

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