OEmail Deliverability Glossary

Open Rate

The percentage of delivered emails that recipients opened. Inflated since Apple MPP (2021) — use alongside reply rate and inbox placement for accurate deliverability signals.

The open rate is the percentage of delivered emails that recipients opened.

Formula: Open Rate = (Unique Opens / Emails Delivered) × 100

The Apple MPP problem:

Since Apple's Mail Privacy Protection (MPP) launched in 2021, Apple Mail pre-fetches email tracking pixels — registering "opens" for all Apple Mail users regardless of whether they actually read the message. This has inflated industry open rates significantly and made open rate unreliable as a standalone engagement metric.

Open rate as a warm-up signal: During warm-up, Inboxwarm.ai network accounts open 100% of warm-up emails, creating a strong open-rate signal. This teaches ISPs that your emails consistently generate high engagement.

Better metrics to track alongside open rate:

Reply rate (unaffected by MPP)
Click-through rate
Inbox placement rate
Bounce and complaint rates

Frequently Asked Questions

How has Apple Mail Privacy Protection affected email open rates?

Apple's Mail Privacy Protection (MPP), launched in September 2021, pre-fetches email content including tracking pixels for all Apple Mail users — registering an 'open' regardless of whether the recipient actually read the email. Since Apple Mail accounts for 40–50% of email opens globally, this has inflated industry open rates by an estimated 20–30 percentage points. Many senders now see open rates of 60–80% that would have been 40–50% before MPP. As a standalone metric, open rate is now unreliable — use it directionally alongside reply rate, click-through rate, and inbox placement rate for accurate deliverability assessment.

Should I still track open rates during warm-up?

Track them, but treat them as a directional signal rather than a precise metric. During warm-up, Inboxwarm.ai's network opens 100% of warm-up emails — creating a strong open signal for ISPs — but the open rate metric you see in your dashboard reflects this artificial 100% rate, not real engagement. What matters is that the ISP sees these opens and counts them toward your reputation. For assessing how your real-world campaigns will perform post warm-up, inbox placement rate (measured via seed testing) is a better predictor than your warm-up open rate.

What open rate should I expect from a warmed domain?

Post-warm-up open rates for cold email typically range from 30–60% depending on your audience, subject line quality, and inbox placement rate. For marketing email to opted-in lists, 20–40% is average for well-deliverable senders. These numbers are inflated by Apple MPP — real 'read' rates are likely 30–40% lower than tracked opens. More reliable benchmarks for a successfully warmed sender are: inbox placement above 90%, reply rate above 2–3% for cold email, and click rate above 2–5% for marketing email. These metrics are harder to fake and better correlate with actual campaign effectiveness.

Related Terms

Reply Rate

The percentage of delivered emails that received a direct reply. The gold standard engagement signal — a reply proves a real human read and responded to your message.

Inbox Placement Rate

The percentage of sent emails that land in the recipient's primary inbox. More meaningful than delivery rate — a message in spam is functionally invisible.

Email Engagement (Warm-Up Engagement Signals)

The recipient actions ISPs use as proof that your emails are wanted — opens, replies, clicks, and spam-to-inbox moves are the most powerful positive signals.

Gmail Postmaster Tools

Google's free service that shows domain reputation, spam rate, authentication pass rates, and delivery errors for mail sent to Gmail. The most important warm-up monitoring tool.

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