Track Email Impressions


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Email service like Mailchimp can provide stats like how many time an email is opened, click rate, who open/click it, etc.


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They embed a 1x1 image that is served from their server. Parameters in the URL are unique to the email, so when that image is requested, they know what email it was in.

Not all email clients download hosted images by default though (for example Gmail by default does NOT unless the user asks to).