IP Address Reputation Primer

There has been a lot of recent discussion and questions about reputation, content and delivery of email. I started to answer some of them, and then realized there weren't any basic reference documents I could refer to when explaining the interaction. So I decided to write some.

This post is about IP address reputation with some background on why IPs are so important and why ISPs focus so heavily on the sending IP.

Why IP addresses?

ISPs built reputation around IP addresses because it was one bit of data that malicious senders / spammers couldn't forge. The connecting IP is a fundamental part of the network transaction and if you forge an IP then SMTP can't work. Because that was the reliable data they had to work with, that's what they used. Even now, when there are other kinds of data, the IP address is still the first thing the receiving MTA sees.

What is IP reputation?

IP reputation can best be summed