newmail: 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/newmail - from package: newmail

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

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