rt-mailgate: 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 CentOS possible default full paths are:

/usr/sbin/rt-mailgate - from package: rt3-mailgate

You may install the required package with command yum install {package}

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