Email Deliverability is the front-line metric for email marketers. Because there’s no chance for a recipient to open, read, and respond to any message if that email never arrives.

So, it’s vital that commercial senders:

  • Understand best practices for actually reaching their customers’ inboxes.
  • Have timely, accurate tracking and reporting of the actual inbox performance of their marketing and transactional emails.

Reaching The Inbox: Bedrock Best Practices


This subject occupies entire textbooks and careers. Our few paragraphs here can only provide the broadest strokes of what’s important. For greater detail see SparkPost’s Deliverability Declassified Guide.

Each item below addresses a major cause of emails ending up in the spam folder.

    • Require email active opt-in. No shortcuts permitted here: no pre-populated opt-in boxes; no hiding of automatic opt-in language in the small print of “Accept Terms and Conditions.” The subscriber must want to sign-up, and know that she’s done so.
    • We emphatically recommend against the use of purchased or rented email lists.
    • Make sure that all new addresses are verified and correct. Once that’s achieved, best practice for recipient validation is sending confirmation/welcome emails immediately upon sign-up


    • Per Forrester Research: “Cover the Basics: Tracking inbox forensics, maintaining a unique sending IP and adopting authentication standards like sender policy framework (SPF), domainkeys identified mail (DKIM), or domain-based message authentication, reporting and conformance (DMARC) still have the greatest influence over your . . . inbox rate.”
    • Clean-up and remove inactive subscribers, or those that have not opened an email in three to six months.
    • Immediately remove all unsubscribes and bounces from your mailing lists.


    • Critical to inbox performance is recent user engagement. Send emails that your subscribers want to receive, based on their known location, status, preference and browse/purchase behavior.
    • Push message content geared to fostering longer term subscriber engagement and retention, rather than connecting solely around promotional events.
    • Don’t over-mail. Test and manage contact frequency based on actual subscriber engagement. In fact, subscribers with decreasing engagement are less likely to unsubscribe if their frequencies are reduced. Higher frequencies should not be used to combat lower engagement Frequency increases should occur over time, rather than suddenly. 

Understanding the “Deliverability” Metric

Deliverability seems straightforward. If you send 1,000 emails, and the email server’s log files say 900 of those were accepted by receiving systems, then your deliverability is 90%. Simple, right? Actually, no. What that figure represents is really the message acceptance rate. That’s because all the server knows is that the receiving system took the message, but not what happened after that. Did it go to the inbox? Or to the spam folder? The sender doesn’t know, because in both cases, the transaction is logged as successful. This ambiguity is why simple server-side measures of message acceptance are inadequate and misleading.

What you really care about is inbox placement. because that’s the only place where a recipient is actually going to see your email. This is what Forrester meant (above) by ‘tracking inbox forensics.”

Essential to this process is having the right data. In the early years of commercial email, senders could use seed-based monitoring of inbox performance. The sender would establish special “seed” email addresses representing various ISPs among the recipients of its campaigns. Those seeds are then monitored to determine where their email landed in their seed account—the inbox, the spam folder… or if it didn’t arrive at all. Because seeds employ a small set of addresses to track, their information is not statistically projectable. Seeds therefore don’t provide a reliable assessment of inbox performance.

Worse, seeds are also inert. They don’t behave. And because email engagement behavior has become such an important factor in how the ISPs manage their spam filters, it’s critical for any inbox monitoring process be based on the behavior of the mailer’s actual email recipients. That’s where another way to measure deliverability performance really becomes important: panel data. Panel providers monitor real-world recipient inboxes (the owners of these mailboxes have agreed to participate in the monitoring) and aggregate data about message characteristics and performance over time. Thus, while seeds can suggest inbox performance, panel data are best for reflecting actual performance, including overall message volume by sender and aggregate inbox placement across campaigns and time.

SparkPost’s consumer panel provides the most expansive coverage on the market today, capturing over 90% of all commercial email. It is comprised of over 1.5 million active and real consumers. These are actual subscribers on your email lists. We currently have subscribers at 42 global ISPs including Outlook, AOL, Yahoo, Time Warner, Comcast, Apple Mail, and many others.

But it’s not an “either/or” proposition. Both seed and panel inbox monitoring methods have useful roles in the tracking process. Your goal here is achieving wide and representative coverage which most accurately reflects real-world activity.

Panel data support statistically valid inbox monitoring for all but the very smallest mailers. The panel’s trackable representation is almost always more accurate than the relatively few seeds usually available to any given mailer. Where that’s not true, seeds can be deployed to supplement panel representation.

Even so, a conventional seed is still a “fake” account which does not behave, and therefore cannot truly represent actual email subscribers.

But, as a technological breakthrough, what if seeds were brought to simulated “life,” to exhibit a representative variety of reality-based locations, attributes and behavior; e.g., reads, deletes, forwards, clicks, replies, purchases)?

SparkPost has such functionality. Called the IntelliX AI Network, this innovation uses machine-learning to apply data representing more than five million actual users. The system deploys inboxes which model user profiles that are geographically, demographically and sociographically representative of actual users. These inboxes’ range of reality-based simulated behaviors can yield far more realistic brand send-patterns, and thus far more accurate deliverability monitoring, than any conventional seed-based monitoring.

Result? With these distinct, independent tracking methods (Panel, IntelliSeeds and conventional seeds), we can — uniquely in our space — cover 43 ISPs, and track 190,000 brands and ten billion searchable emails with the highest degree of accuracy. Sitting atop the resulting data platform is the industry’s most robust and actionable tracking and analytics functionality for our Inbox Tracker product. None of our competitors has this breadth of coverage and functionality, because none of our competitors has the supporting data assets.

Wrapping Up: For your email to have its intended business impact, it must reliably reach your subscribers’ inboxes. Solid inbox performance requires that you:

  • Ensure inbox reputation and subscriber engagement and retention by observing best practices related to sender authentication, audience acquisition and message optimization
  • Have the right data to maintain timely, accurate and actionable inbox tracking and diagnostics

We know how. Our clients know this. Their inbox performance proves it.