This is very true. I can watch my firewall processing email at any given time and many of the connecting hostnames appear to be coming from broadband internet subscribers.
"My mail server gets a request from a sending host, records the IP address, then boots the email back with a 450 error.
Very clever, sending a 'soft error' in response to an SMTP connect. That will slow things down a bit, but if a quick response isn't important in your operations that's all good.
It's only the very first email from each mail server that gets slowed down. After a box is verified to be a real mailserver, it goes into the whitelist. Entries in the list expire every so often. I think the default is 30 days.