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The emails were sent via Mailchimp, the open rate was gotten from Mailchimp's reports (analytics).

Yep indeed it's faulty: https://blog.mailchimp.com/how-gmails-image-caching-affects-...

I wouldn't say its faulty. They clearly said unique open rate is even more accurate now. The only thing that suffered was seeing multiple opens from same user. Which I feel is far less interesting

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