My customer wants block sending email.
They want use something like, one sender can sends few e-mail per minutes, e.g. 100 emails/minutes.
Have you ever met with such a possibility in Email Gateway?
Email > Email Configuration > Protocol Configuration > Protocol Settings(SMTP) > Denial of service protection section. There is an option for "maximum number of recipients before a failure response is given." Change it from no limit to whatever you would like for the max recipients. There are a couple of things to look out for on this. If you do not want to do this globally, use protocol presets. The failure response given is a 400 level error, so a legitimate sending server will attempt to deliver to the rest of the recipients based on its retry schedule.