One thing I can see this not working considering your request is:
Some providers have smart hosts used as their own email servers, and if a global setting had been used like 50 emails/hour/IP it would render some email servers unusable as after 50 emails were sent via smart host, the email server would have been "blocked" (you can add ip to ignore list, but this is not the point)
An email server with tight sensing email restrictions makes sense to stop spammers.
A smart host, if self-hosted, should probably have restrictions disabled "unlimited".
Other things should probably be taken into consideration, but this was the most obvious.
