rt-mailgate-4: 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/rt-mailgate-4 - from package: rt4-clients

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

Error in other languages:
rt-mailgate-4: bevel nie gevind nie
rt-mailgate-4: comandă negăsită
rt-mailgate-4: comando não encontrado
rt-mailgate-4: commande introuvable
rt-mailgate-4: command not found
rt-mailgate-4: käsku ei ole
rt-mailgate-4: không tìm thấy lệnh
rt-mailgate-4: komanda nerasta
rt-mailgate-4: Kommando nicht gefunden.
rt-mailgate-4: kommandot finns inte
rt-mailgate-4: komut yok
rt-mailgate-4: nie znaleziono polecenia
rt-mailgate-4: níor aimsíodh an t-ordú
rt-mailgate-4: no se encontró la orden
rt-mailgate-4: no s'ha trobat l'ordre
rt-mailgate-4: opdracht niet gevonden
rt-mailgate-4: parancs nem található
rt-mailgate-4: perintah tidak ditemukan
rt-mailgate-4: príkaz nenájdený
rt-mailgate-4: příkaz nenalezen
rt-mailgate-4: команда не найдена
rt-mailgate-4: командата не е открита
rt-mailgate-4: 命令找不到
rt-mailgate-4: コマンドが見つかりません