Mail Rate Limiting

In the Mail Rate Limiting options, I wondered if there is a way to have more than just 2 “Per sender domain overrides”? There doesn’t seem to be a way to add more form fields for these, but 2 seems arbitrarily low?

The problem I’m having (perhaps I’m going about it the wrong way) is that I have a handful of trusted domains (5 or so) and expect about 150-300 messages (in/out) daily. I tried setting the “Global message limit” to: 500 every 1 days. But I’m also getting a lot of attempts on this system (sometimes 500 a day) from other unauthorized senders… these are getting rejected by the server, but when that happens, I have problems sending mail from my legitimate handful of domains.