CTOR is computed as unique clicks / unique opens over the same campaign or flow. It sits between open rate and click-through rate. Open rate (opens / sends) measures subject line and sender-name strength; click-through rate (clicks / sends) measures end-to-end campaign performance; CTOR uses opens as the denominator to isolate the in-email layer — creative, CTA copy, and offer placement — given that the recipient opened.
The post-MPP read is more careful. Apple’s Mail Privacy Protection, introduced in iOS 15 (September 2021), pre-fetches tracking pixels for Apple Mail users who opt in, inflating and de-timing opens — part of the same privacy-era measurement-degradation pattern as ATT. That inflation hits CTOR’s denominator, so absolute CTOR numbers are noisier post-MPP. The metric still earns its place: list quality and subject-line performance are already factored out, leaving CTOR as the cleanest available read on creative and CTA strength inside the email.
CTOR is the right diagnostic in three situations. A/B tests of email creative — hero image, CTA copy, offer placement — use CTOR to isolate the change from list and subject-line noise. Flow comparison across welcome, abandonment, and winback uses CTOR to isolate flow-content quality from flow-trigger volume. Segment-level reads treat a low CTOR on an engaged segment as a signal of creative fatigue rather than list decay. Read CTOR relationally against your own baseline, not a published “good” number — vertical, list, and flow type shift the absolute level enough that any benchmark misleads.