Why open rate stopped being a reliable metric
Email open rate has historically been measured by a tracking pixel that loads when a recipient opens the message. Apple Mail Privacy Protection preloads that pixel automatically for every Apple Mail user, whether or not the recipient ever opened anything — the “opened” event fires either way. That isn’t a minor accuracy hit: it’s a metric that no longer measures what it claims to, across a share of traffic that isn’t small given how much of the corporate inbox runs on Apple Mail.
Continuing to optimize an outbound sequence for open rate, with that distortion baked in, is optimizing against noise.
The principle that replaces it: RPM
RevSign’s internal doctrine defines RPM (Revenue Per Message) as attributed ARR divided by the number of messages sent. The goal is to maximize RPM by filtering out low-value signals before sending — not sending more, sending better.
Today this is a design principle that shapes how a sequence gets filtered and prioritized (see Validation Pipeline for the threshold rule that decides which signal turns into an outreach), not a dashboard with an implemented, customer-facing attribution formula. It’s worth being precise here: it’s an explicit design goal, not yet a calculated and reported metric.
Unsubscribe rate as a relevance signal
The same doctrine treats unsubscribe rate as the most honest indicator of whether a message had enough context. An unsubscribe isn’t just a lost contact — it’s a signal that the message arrived without the context that would have justified it. It’s a harder metric to game than open rate: it doesn’t depend on a pixel, it depends on a deliberate action from the recipient.
Open rate vs. RPM vs. unsubscribe rate
| Metric | What it actually measures | Gameable by | Decision it enables |
|---|---|---|---|
| Open rate | Less and less: image preloading distorts the count | Privacy-protecting mail clients, with no recipient intent involved | Nothing reliable |
| RPM (Revenue Per Message) | Attributed ARR per message sent | Hard to inflate without generating real revenue | Which signals are worth turning into a sequence, which to filter before sending |
| Unsubscribe rate | Perceived relevance of the message to its recipient | Requires a deliberate action, harder to game | Whether a sequence needs more context before going out |
How this connects to the rest of the system
The threshold and intensity filter described in Validation Pipeline is, in practice, this principle’s implementation: a high-impact sequence only fires when the intent score crosses a high band, not when “there’s a list to burn through.” Fewer, better-filtered messages is the only real lever available today to move RPM in the right direction.
$ curl revsign.os/docs/learning-loop-and-rpm
→ {
title: "Learning Loop: Why Open Rate Stopped Being an Outbound Metric",
answers: "What metrics replace open rate in modern outbound?",
schema: [TechArticle]
}