Key Takeaways

  • Deliverability cannot be solved with more servers or faster compute; mailbox providers throttle traffic based on recipient engagement rather than outbound sending bandwidth.
  • When sending a blast to 100,000 recipients, sort the queue to deliver to your most engaged subscribers first so early opens and clicks warm up inbox provider algorithms.
  • To protect domain health, separate transactional and promotional traffic across dedicated subdomains rather than blasting everything from an apex domain.
  • Link tracking domains must match your verified sender domain, as mismatched tracking redirects trigger automated phishing and abuse filters.
  • Traditional open rate metrics have become unreliable because security scanners and Apple privacy features automatically pre-fetch tracking pixels.

The Method

Software engineers often treat email deliverability as a classic pipeline problem: if you need to send 100,000 emails, you provision more worker instances, spin up Redis queues, and blast them out as fast as your network interface allows. Mailbox providers treat that behavior as hostile. As Rocha explains, “With email, you got to respect the rules of the game and how Gmail behaves is differently from Outlook and differently from Yahoo mail.” (play from 7:29)

To pass mailbox provider filters at scale, you have to reverse-engineer how abuse prevention systems classify bad actors. Rocha summarizes the operating principle simply: “The whole way of thinking about email marketing and email in general is you need to do exactly what an abuser wouldn't do. It's the opposite of that.” (play from 10:48)

First, re-order your outbound queue around user activity rather than database insertion order. When you dispatch a broadcast to 100,000 contacts, Gmail and Outlook watch the initial batch to decide whether to route the rest to the inbox, send them to the spam folder, or bounce them. “There are clever things you can do where 100,000 people put the most engaged audience members in the beginning of that queue,” (play from 9:27) Rocha notes. When your initial 5,000 emails show immediate opens and replies, mailbox algorithms grant a temporary reputation boost, clearing a path for the remaining 95,000.

Second, isolate email reputations by purpose. Rocha points out that reputation has moved from IP addresses to domains: “Now there's a lot of reputation associated with the domain. So then what people do is you create different subdomains depending on what type of emails you're sending.” (play from 56:13) Use a dedicated subdomain for password resets and receipts (such as auth.yourdomain.com), and keep marketing updates on a completely separate subdomain (such as news.yourdomain.com). A promotional campaign that gets marked as spam will never delay an authentication code.

Third, verify your click tracking alignment. Many third-party email tools wrap links in generic redirect domains. When a spam filter sees an email signed by your brand routing users through a shared tracking domain, it flags the message as potential phishing. Align your CNAME records so every tracked link uses your custom subdomain.

Where This Breaks Down

This method breaks down when teams rely on raw open rates to gauge engagement. Modern email security tools and Apple Mail Privacy Protection routinely pre-fetch assets, firing tracking pixels before a human ever touches the message. Rocha warns: “The open rate as a metric as a whole isn't really as reliable as it used to be because all these spam filters are checking that before.” (play from 14:09) If you define your most engaged users solely through open pixels, your sorted queue will prioritize corporate firewalls and automated bots instead of real buyers.

Additionally, queue sorting cannot rescue a domain that has already suffered high complaint volumes. If your list contains unverified scraped contacts, mailbox providers will flag your domain before the tail of your list ever leaves the queue.

What to Do With This

Audit your DNS and outbound email pipeline this week. First, split your sending domains by routing transactional emails through a subdomain like notify.yourdomain.com while moving promotional campaigns to updates.yourdomain.com. Second, update your broadcast sending query to sort subscribers by recent link clicks or site activity within the last 30 days rather than account creation date, ensuring your most active users receive the first batch of any mass send.