grepmail: 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
/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/grepmail - from package: grepmail

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