Email Open Rate
Also known as: email engagement rate, email open rate, open rate (email)
Email open rate is the percentage of delivered emails that recipients open, calculated as (opens / emails delivered) x 100.
It has historically been the primary email marketing engagement metric, indicating whether subject lines are compelling and whether the sending list is engaged. However, Apple's Mail Privacy Protection (iOS 15, 2021) pre-loads email content including tracking pixels on Apple servers regardless of whether the user opened the email, inflating open rates for Apple Mail users and making open rate data unreliable as an absolute metric. Click-through rate (CTR) has become the more reliable primary engagement metric for email campaigns.
Email Open Rate, explained at three levels
At home
Email open rate tells you what percentage of people who received your email actually opened it. If you send 1,000 emails and 250 people open them, your open rate is 25%. It's the most basic measure of whether your emails are compelling enough for recipients to open them, starting with your subject line and sender name: those are the only things visible before a recipient decides to open or ignore an email.
At work
Businesses use email open rates to judge whether their email list is engaged and whether subject lines are working. A declining open rate across multiple campaigns suggests the list is becoming less engaged (subscribers no longer interested but not unsubscribed), the subject lines are becoming less compelling, or the emails are landing in spam. Open rate is the first funnel metric for email: people who don't open can't click, and people who don't click can't convert.
At home
Open tracking relies on a hidden image. When the platform sends a campaign, it inserts a tiny image whose web address is unique to each recipient. When the email client loads images, it requests that address, and the platform logs an open for that person. Open rate is unique opens divided by delivered emails. The method has blind spots: clients that block images undercount opens, while Apple Mail Privacy Protection loads images automatically and overcounts them. Real opens are driven by sender name, subject line and preview text.
At work
Apple Mail Privacy Protection (MPP) has made email marketing metrics more complex. Most email platforms now segment open data by mail client and label Apple MPP opens separately. Marketers are adjusting strategies: using click rate as the primary engagement metric (clicks require actual human intent), testing subject lines by looking at click-rate differences rather than open-rate differences, and using 'click-to-open rate' (CTOR: clicks as a percentage of opens) as a proxy for content quality among genuine openers.
At home
Email deliverability is directly affected by engagement rates: email providers (Gmail, Outlook) monitor open and click rates as signals of whether recipients want your emails. Sending to a large unengaged list (people who haven't opened in 12+ months) generates low engagement signals, increasing the probability that Gmail classifies your emails as promotional or spam for the entire list. Deliverability management (suppressing unengaged subscribers, segmenting by engagement, maintaining bounce and unsubscribe list hygiene) protects sender reputation.
At work
Predictive send-time optimization (machine learning in email platforms like Klaviyo, Marketo, Salesforce Marketing Cloud) analyzes each subscriber's individual open and click history to determine the optimal send time for that specific person, deploying a broadcast email across a staggered schedule that maximizes individual likelihood to open. List-based sender reputation management at email service providers involves monitoring bounce rate (below 2% target), unsubscribe rate (below 0.1%), and spam complaint rate (below 0.08%) to maintain inbox placement quality.
Real-world examples
At home
Fatima managed email for her craft supply store. Her open rate went from 28% to 41% after she switched her sender name from 'CraftSupply.ca' to 'Fatima from CraftSupply'. She also started A/B testing subject lines every campaign: 'New Arrivals This Week' vs. 'The thread colours everyone is talking about': the second style consistently won by 15-20 percentage points in the A/B test.
At work
Sophie was a B2B email marketer whose open rate jumped from 22% to 58% overnight after iOS 15 launched: clearly MPP inflating the numbers, not genuine improvement. She rebuilt her reporting to focus on click rate and conversion rate instead. She implemented engagement scoring (subscribers who had clicked in the last 90 days) and suppressed non-engagers from broadcast campaigns, sending them a re-engagement sequence instead. Deliverability improved and click rates from the active list increased.
Common confusions
- Many email marketers still treat open rate as a reliable indicator of email performance — Since Apple Mail Privacy Protection launched in 2021, open rates for lists with significant Apple Mail usage are inflated and unreliable as an absolute metric. A reported 55% open rate may actually represent 25% genuine opens plus 30% MPP machine-opens. Click rate, click-to-open rate (CTOR), and conversion tracking are more reliable engagement metrics post-MPP. Open rate is still useful for relative comparisons (subject line A/B tests) but requires careful interpretation as an absolute number.
- Some believe a high email open rate indicates the campaign was successful — Open rate measures whether people opened the email, not whether they took any action. A campaign with 40% open rate and 0.5% click rate may generate fewer leads than one with 25% open rate and 4% click rate. Open rate is one funnel step: people must also click, land on your page, and convert for the campaign to generate business value. Measure the full email-to-revenue funnel to evaluate campaign success, not just the first step.
For analysts
When a client cites increasing email open rates as evidence of improving email performance, ask when the increase started and what percentage of their list is on Apple devices. A spike after October 2021 that hasn't been followed by corresponding click rate improvement is almost certainly MPP inflation. Redirect the conversation to click rate and conversion rate, and implement proper MPP-aware reporting that segments machine-opened emails from genuine openers.
History & origin
Email tracking began when marketers embedded tracking pixels (1x1 transparent images) in emails: when the email was opened and images loaded, the server was pinged, recording the open. This method worked reliably for most of the 2000s and 2010s. Average B2B email open rates hovered around 20-25%. Apple's Mail Privacy Protection (MPP), launched with iOS 15 in September 2021, fundamentally changed email measurement: Apple pre-fetches and caches all email images (including tracking pixels) when the email arrives on an Apple device, regardless of whether the user actually opens the email. This causes open rates from Apple Mail users to be recorded as 100%, dramatically inflating reported open rates. Since Apple Mail holds a large share of email clients (estimates range from 40-60% of email opens globally), this made open rates unreliable across most email programs.