Thanks to OreoCookie and
MacGeekery here is what you have to do
1. Open terminal
2. type "sudo bash" (to log in as root, needs admin password)
3. type "ipfw pipe 1 config bw xxxkbit/s" (xxx is desired max speed)
4. type "ipfw add pipe 1 dst-port smtp" (throttles smtp to the speed of step 3)
if you want to delete the settings
type "ipfw flush"