whups-mail-filter: 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/whups-mail-filter - from package: php-horde-whups

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

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