Today I was discussing some mailing list posts with an ESP colleague. He was telling me some interesting numbers he’d collected from different IP pools they maintain. He was testing routing mail through IPs based on subscription process and routing based on engagement metrics. The data showed that inboxing rates were similar across the test groups. As he put it, “IP reputation didn’t have much impact on inbox delivery.”
I’m not surprised. I’ve been talking for a while about how IP reputation is less important in reaching the inbox. In fact, it was almost 5 years ago now that I wrote The Death of IP Based Reputation. I updated it in 2015 with Deliverability and IP Reputation. Overall, IP reputation is a much smaller piece of reaching the inbox now than it has been in the past. I’ve talked about the reasons for this in the above posts. The short version is:
- IP reputation is a crude hammer;
- IPv4 addresses are in very limited supply, in network terms more customers / IP is a good thing;
- Spammers use botnets, sending large amounts of email across many IPs;
- IPv6 is huge and IP based blocking will be challenging and of limited effectiveness; and
- Better computing power makes content scanning more feasible.
IP Reputation Still Matters, a little
This doesn’t mean senders can, or should ignore IP reputation. Even Gmail looks at IP reputation a little bit. The place IP reputation is primarily used during the SMTP transaction. Good IP reputation does lead to less rate limiting. Senders with good IP reputation can send more mail faster than senders with poor reputation. But once the SMTP transaction is over, IP reputation is just a small factor in a large pool of variables.
IP Reputation Still Matters, a little more.
There are some places that heavily rely on IP filters. And some places that rely on certain types of IP filters. Most of the major providers will block mail from home users, dynamic IPs, and infected machines. Additionally, there is and will probably always be a long tail of domains that are still relying on IP based filters. It’s a crude hammer, but it’s an effective one. Typically, though, IP reputation in those cases is in the eye of the root user. The good news is, these are often private networks, and users have the option to use less restrictive free providers if they’re not getting the email they want.