Open rate is the percentage of email recipients who open a newsletter issue. It is calculated as: (Unique Opens ÷ Delivered Emails) × 100. In newsletter advertising, open rate is critical because it determines the actual ad impression volume — if your newsletter has 100,000 subscribers and a 35% open rate, each send generates approximately 35,000 impressions (assuming one ad slot per email). However, open rate measurement has become less reliable since Apple's MPP (Mail Privacy Protection), which pre-fetches email pixels and inflates apparent open rates for Apple Mail users. Newsletter publishers should distinguish between 'human opens' (genuine reads) and 'proxy opens' (Apple MPP or Gmail prefetch). MailAdx tracks this distinction in delivery reporting, giving publishers accurate impression data.
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