Any way to limit outgoing email rate by domain?

Some hosting providers somehow limit the number of emails per domain per period of time, so, maybe 500 per hour per domain.

I don’t see a way to do this with postfix. Does anyone have a way or trick to accomplish the same task?

webmin-servers-postfix mail server->right pane => delivery rates?
would that be it?

No, that wouldn’t be it. That does not let you apply any limit to mail from any given domain. Apparently, this is possible, further research shows this can be done by creating a policy server. So, will do.