May 28, 2013

How We Use Engagement to Deliver Email

If you read our deliverability posts closely, you’ll know MailChimp does something called engagement-based routing. It sounds like throwaway jargon that’s meant to impress you without providing any details. However, I’m here to provide some details. Booyah!

(Was that inappropriate? Hmm, moving on…)

You can think of engagement-based routing as a reward for high member ratings. If you’re not familiar, member rating is the five-star metric that balances a subscriber’s open and click data with your sending frequency. Behind the scenes, this scale is based on something called activity score. That’s going to be important later, so you may want to read up on it.

Now for the fun part. Every time you send a campaign, MailChimp looks through the list and pulls out your biggest fans. These subscribers have proven how much they love your content, and they deserve some special treatment, right? We move them to the front of the list and route their emails to special engagement-based IPs. We have two tiers of these special IPs, and we call them "Active" and "Halo" to describe how good the traffic is.

Why does routing matter? Sometimes an ISP sees a lot of bounces or spam complaints, so they temporarily throttle an IP. They can even block an IP for a few hours. However, engaged subscribers don’t bounce and they rarely hit the "spam" button, so IPs that only send engaged traffic don’t have to worry about that stuff. Also, we purposefully under-utilize Active and Halo IPs, so your email goes out as quickly as possible—even during peak hours.

Curious about what it takes to send over an Active or Halo IP? Here’s the actual breakdown between IP tier, activity score, and member rating:


(There, that felt appropriate.)

As you can see, we’re a tad strict about our engagement-based routing. Even some five-star subscribers don’t go out over Halos. Using activity scores to determine engagement gives us the fine-tuned control we need. It may look strict, but as it turns out, 18% of our traffic is engaged enough to receive this special attention. So why don’t we lower the thresholds and let more email go over Halos and Actives? For a few reasons, actually.

First, having rigorous standards for our high-engagement IPs ensures they have the best possible reputation with the ISPs to which you’re delivering. Second, it’s important we send really good traffic over all our IPs. Makes sense, right? Every new subscriber starts out with no engagement, so we need to make sure all of our IPs adhere to MailChimp’s high standards. Finally, if we scoop away all the good traffic, we’d leave those normal IPs susceptible to the occasional bad apple.

In the course of a day, most traffic to two- and three-star subscribers is perfectly fine. But every now and then one of our users slips up and sends some questionable content to a questionable list. Without a little good traffic to provide balance, one bad apple could singlehandedly tank an IP’s reputation. By keeping three- and some four-star traffic in the "normal" zone, we’re giving Omnivore time to find the bad apples without endangering new subscribers for no reason.

Overall, engagement-based routing is one of many small optimizations we make behind the scenes. It’s not going to make or break your campaign, but if you have engaged subscribers, you’re kind of making it on your own anyway. This is just a free upgrade to first class.