What’s the difference between an ESP and an MTA (Mail Transfer Agent)?

Apple MPP inflates reported open rates because Apple's servers pre-load tracking pixels for all emails delivered to users with MPP enabled, regardless of actual viewing. An email that sits unread in someone's inbox still registers as "opened" when Apple's proxy fetches the content. This creates false positives that make open rates unreliable as engagement metrics for any list with significant Apple Mail representation.

The impact varies by audience composition. B2C brands with heavy iPhone user bases may see their open rates jump dramatically while actual engagement remains unchanged. B2B senders with more Windows and Android users experience smaller distortions. Understanding your audience device mix helps you assess how much MPP affects your specific metrics, though even this analysis has become harder as device detection also gets blocked.

Many marketers report open rates 10-30% higher than pre-MPP baselines, but this increase represents measurement noise rather than improved engagement. Comparing current open rates to historical data becomes misleading unless you account for this fundamental change. Open rates haven't become meaningless, but they now require careful interpretation and should never be your only measure of email success.