The measurement gap after iOS 15
Apple's Mail Privacy Protection, introduced in iOS 15 in September 2021, fundamentally changed email open rate measurement. By pre-loading email content and images regardless of whether the email is actually opened, it causes many Apple Mail reads to register as opens in email analytics platforms, inflating open rates for lists with significant Apple Mail audiences. For many email programmes, the reported open rate jumped 10 to 20 percentage points after iOS 15 while actual opens probably changed very little.
The practical consequence is that open rate is no longer a reliable primary metric for email list health assessment. Sending teams that are still using open rate as their main indicator of list health are making decisions based on a measure that has been distorted in ways that are difficult to fully correct for. The list may appear healthier than it is, or the trend over time may look better or worse than reality because the inflation affects different segments of the list differently.
The good news is that email list health is still measurable. The metrics have shifted, but the underlying questions, is this list engaged, is it delivering against our goals, is it likely to produce good deliverability, are still answerable with the right data.
Click-through rate as the primary engagement measure
With open rate compromised as a primary metric, click-through rate has become more important as an engagement indicator. Clicks are real actions that Apple's pre-loading does not fabricate: a click requires a human to make a deliberate decision to engage with a specific link. Click-through rate, defined as the percentage of delivered emails that receive at least one click, is a meaningful measure of active engagement with the content.
Click-to-open rate, which measures clicks as a percentage of emails that registered an open rather than as a percentage of total deliveries, is still used by some teams but inherits the open rate inflation problem and is therefore less reliable than raw click-through rate as a current benchmark.
For most email programmes, a healthy click-through rate on content emails is 2 to 5%. Promotional emails often perform at the lower end. Highly targeted nurture emails with strong relevance to the recipient's current situation can perform significantly above this range. The benchmark matters less than the trend: is click-through rate stable, improving, or declining over time?
Clicks are real. Opens, post-iOS 15, are not always. Build your list health assessment around the actions that require a human decision, not those that can be machine-generated.
Conversion rate as the commercial health measure
The metric that most directly tells you whether the email programme is commercially healthy is conversion rate: the percentage of email sends that result in a desired action beyond the email itself, a purchase, a booking, a registration, a download. This metric connects the email programme directly to the business outcomes it is supposed to serve.
Conversion rate varies enormously by programme type, audience, and objective. The most useful version is trend analysis within the programme over time: is the conversion rate stable, is it improving with optimisation work, or is it declining in ways that need investigation? A declining conversion rate on a consistent programme is often the earliest signal of list fatigue, content misalignment, or audience drift that will eventually affect deliverability if not addressed.
Deliverability signals
Email deliverability, whether emails are actually reaching the inbox rather than the spam folder, is foundational to list health but not always monitored. The signals that indicate deliverability problems include: hard bounce rate above 2%, soft bounce rate trending upward over time, spam complaint rate above 0.1%, and a significant proportion of sends going to "inactive" domains that are often associated with spam traps.
Most email platforms provide deliverability data in some form. Reviewing these metrics monthly and investigating significant changes promptly protects the sender reputation that determines whether future sends reach the inbox at all. A sender reputation problem is significantly harder to recover from than to prevent.
List hygiene as a health maintenance practice
A list that has not been cleaned in twelve or more months is a list that contains a meaningful proportion of inactive, invalid, and potentially spam-trap addresses. Sending to these addresses does not just waste the send; it actively damages sender reputation by increasing bounce rates and spam complaint rates relative to the size of the genuinely engaged list.
A quarterly list hygiene process that removes hard bounces promptly, identifies contacts with no engagement activity in twelve months, and either re-engages them through a specific campaign or removes them from the active list, maintains deliverability and produces more accurate engagement metrics from the remaining list. A smaller, actively engaged list consistently outperforms a larger, partially engaged list on every metric that matters commercially.
Building the measurement habit
Email list health is not a project; it is an ongoing measurement responsibility. Building a monthly review of the four or five metrics that matter most, click-through rate, conversion rate, bounce rates, spam complaint rate, and list growth rate, into the regular marketing reporting rhythm ensures that problems are identified early and addressed before they compound. An email programme that is reviewed monthly and maintained proactively is substantially more valuable over time than one that is only audited when a problem becomes obvious.