Open rate is calculated as unique opens divided by delivered emails, expressed as a percentage. For decades, it was the primary metric for email subject line performance and audience engagement. That changed in September 2021 when Apple introduced Mail Privacy Protection (MPP).
MPP pre-loads email content (including tracking pixels) for Apple Mail users, registering an "open" even if the person never actually looked at the email. Since Apple Mail accounts for roughly 50% to 60% of email opens in many B2B databases, this inflated open rates across the board and made the metric unreliable as a standalone measure.
Despite its limitations, open rate still has some value. Relative comparisons (this month vs. last month, Campaign A vs. Campaign B) can still reveal trends, as long as the Apple Mail proportion of your audience remains roughly constant. It also still works for non-Apple recipients, which you can segment for if your MAP supports it.
The practical shift for MOps teams has been toward click-based engagement metrics. Click-through rate and click-to-open rate are more reliable indicators of genuine engagement because they require an affirmative action from the recipient that cannot be faked by privacy features.
If you still report on open rates, contextualize them. Note the percentage of your audience on Apple Mail, show the trend rather than the absolute number, and pair open rate with click rate data. Better yet, shift your reporting to metrics that reliably indicate engagement: clicks, replies, and downstream conversions.