What’s the proper way to interpret CTOR after MPP?
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Before Apple Mail Privacy Protection, click-to-open rate was a reasonably clean content quality signal. Now it's noisier, but it's not useless. You just have to know what's happening under the hood.
MPP pre-fetches tracking pixels automatically for Apple Mail users. This inflates your unique open count, which is the denominator in the CTOR formula (clicks divided by opens, times 100). If machine-opens push your apparent open count from 500 to 1,500, the same 80 real clicks now show a 5.3% CTOR instead of 16%. The content didn't change. The denominator did.
How to read CTOR in the MPP era:
- Don't trust absolute numbers. An 8% CTOR doesn't mean the same thing it did in 2020. What matters is direction and relative comparison.
- Compare campaigns to each other, not to pre-2021 baselines. If CTOR is trending up across your recent sends, that's a real signal. If you're comparing to numbers from three years ago, you're comparing apples to a different fruit.
- Segment by mail client if your ESP supports it. CTOR for non-Apple Mail readers gives you a cleaner read. That cohort doesn't have MPP inflation in the denominator.
- Watch CTR alongside CTOR. If CTR is holding steady but CTOR is dropping, that's more likely an inflation artifact than a content problem. Real engagement deterioration shows up in both.
The metric isn't broken. It's just noisier. Use it for direction, not as a precise score.
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