The short definition
Open rate = emails recorded as opened ÷ emails delivered.
Opens are usually detected by a tiny invisible image loading when the email is displayed. If the image loads, the platform records an open.
Why it has become unreliable
That mechanism depends on the recipient's mail client loading the image. Some now pre-load images automatically for privacy reasons — the image is fetched whether or not the person ever looked at the message.
The result is inflated and inconsistent open rates: some recipients register an open they never made, and others who genuinely read the email register nothing because images were blocked.
This is not a small distortion, and it varies by which mail client your audience uses — which you do not control.
What to use instead
Clicks. A click is a real action by a real person. Click rate is far more reliable than open rate and always was.
Replies. For a local service business, the strongest signal there is. An email that produces replies is working, whatever the platform reports.
Enquiries and bookings after a send. The only measure that connects to revenue. Watch what happens to your enquiry volume in the days after a campaign.
Unsubscribes and complaints. Still meaningful, and worth watching closely — a rise in either indicates a real problem.
Where open rate is still useful
As a relative comparison over time on the same list, watched for large movements rather than small ones.
A sudden collapse can genuinely indicate a deliverability problem — mail landing in spam rather than the inbox. That is worth investigating.
What it cannot do is tell you whether a subject line was good, which is what it is most often used for.
The subject line question
If you cannot trust open rate, how do you judge a subject line?
Honestly: mostly you cannot, at small volumes. Which is an argument for writing subject lines that are plainly informative rather than optimised — specific, short, and describing what is inside.
"Your AC before July" tells the reader what this is. That is the whole job.
The practical position
Do not report open rate as a headline number and do not make decisions on it. Watch clicks, replies and enquiries.
Checking your own
Compare open rate against clicks across your last few sends. If opens move and clicks do not, the opens are noise.
Frequently asked questions
Why is open rate unreliable now?
Opens are detected by a tracking image loading. Some mail clients pre-load images automatically for privacy, registering opens nobody made — while others block images, missing genuine reads.
What should I measure instead?
Clicks, replies, and enquiries in the days after a send. Replies are the strongest signal for a local service business, and enquiries are the only measure that connects to revenue.
Is open rate ever useful?
As a relative comparison over time on the same list, watching for large movements. A sudden collapse can indicate a deliverability problem worth investigating.
How do I judge a subject line then?
At small volumes, mostly you cannot — which argues for writing plainly informative subject lines. "Your AC before July" tells the reader what is inside, which is the whole job.