There has been a lot of recent discussion and questions about reputation, content and delivery. 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 first 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 up as “past performance is an indicator of future results.” In other words if recipients responded well to mail from an IP address in the past, then they’re likely to respond well to new mail from that IP address.

How is IP reputation measured?

While each spam filtering company and ISP have their own ways of calculating the reputation of an IP address, there are some similarities in what they measure.

• How many non-existent email addresses is this IP attempting to deliver to?
• How many abandoned email addresses is this IP attempting to deliver to?
• How many “known bad” email addresses (spamtraps) is this IP attempting to deliver to?
• How many recipients complain about receiving this mail?
• How many recipients complain about not receiving this mail?
• How respectful of my resources is this IP?
• Does this IP keep connections open for long periods of time?
• Does this IP retry deliveries too aggressively?
• Does this IP stop mailing addresses after receiving a “user unknown” message?
• Is this IP address configured as if the associated machine was infected by a virus?
• Is this IP address listed on blocklists we use?

That is by no means an exhaustive list of what ISPs measure. If they can measure it they’ve tried. If the measurement helps them separate spam mail from not-spam mail then they’re using it.

How fast does IP reputation change?

IP reputation is often measured over multiple time periods. ISPs can look at a 1 day, 7 day, 30 day and 90 day reputation. A good analogy is stock prices. Prices can be very volatile in the short term, but more consistent over the long term. A single bad day, where one or more reputation measurements go bad, may affect delivery that day or the next day but won’t damage an overall good reputation. Likewise, a few days of improved mail may not be sufficient to counter months of poor reputation.

How is IP reputation used?

Mail from IPs with a high reputation is accepted faster and at a higher rate than mail from IPs with a lower or unknown reputation.  IP reputation can also influence whether mail is delivered to the inbox or the bulk folder.

Key IP Reputation takeaways

• IP reputation is about how recipients react to mail from that IP. Happy, content recipients turn into good delivery.
• Brief changes (for good or bad) don’t necessarily ruin delivery over the long term.
• Steady improvements will result in improved reputation.
• It may takes as much time to change a reputation in one direction or another as it took to establish the reputation in the first place.

Next we’ll look at content reputation, how it’s measured and used.
EDIT: A version of this information is available at the Word to the Wise wiki
EDIT: This post was also shared at CircleID

ip addresses • IP Reputation • primers • recipient • Reputation