IP Reputation is the newest tool in the fight to curb spam. IP reputation is different than an IP blacklist.
One of the large IP reputation is Cisco. They started a group called IronPort or senderbase.org. Although the entire technology used is not known it is clear that they monitory email volumes sent from any server that sends email. Based upon this volume your IP reputation is set. Senderbase.org uses Good, Neutral or Poor. If your IP reputation is set to poor then your emails are blocked and never delivered.
Everyday we are asked "Can I send 300,000 or a million emails a day?" The answer is simple. YES you can send that many but your IP reputation will be set to "POOR" by the end of the day.
So.....what is a safe amount. As far as we can tell that limit is around 70,000 per day. Maybe even less and no one knows for sure.
How is IP Reputation set?
It is simple. All email must originate from an IP address, and IP reputation can be used to tell if a certain IP Address is responsible for sending Spam or Unwanted Bulk Email (UBE). And it is extremely effective, stopping between 80-95% of all inbound connections at ISP's and sometimes more.
It is up to the person who owns that IP to be responsible for what comes out of that IP Address, and if they don't stop spam from originating they can get a reputation for that behavior. Almost every large company, ISP and email server uses it to some extent. Otherwise they would have to process almost 20 times as much email as they would without it.
And there are many companies and resources that track that type of behavior, and create IP reputation lists. Often those lists are used to completely reject email coming from that IP Address. And these lists are compiled from data that shows that IP Address is responsible for only certain types of behavior.