Open rate

Open rate is one of the core metrics in email marketing. It measures what percentage of delivered messages recipients opened and is used to judge whether the subject line and sender name are effective at prompting people to look inside.

How open rate works

The metric is calculated as the ratio of unique opens to messages delivered (not sent — bounces are removed from the total). Technically an open is recorded by a tiny transparent image, known as a tracking pixel, embedded in the email body: when the mail client fetches it from the sender's server, the system counts the message as opened.

This mechanism has limits, though. If a mail client blocks image loading, the open is not counted even though the recipient read the message. The opposite problem arrived with Apple Mail privacy protection, which pre-fetches images on Apple's servers — artificially inflating opens. That is why open rate is now treated as a rough signal, complemented by click-through rate and conversions.

Practical application

Open rate is used above all to test subject lines and send times — in A/B tests two subject variants are compared to see which drives more opens. It also helps gauge list health: a falling rate can signal list fatigue or problems with deliverability.

Many factors shape the result: the appeal of the subject line and sender name, the reputation of the sending domain, the hygiene of the mailing list, and correct authentication (SPF, DKIM, DMARC) that decides whether a message reaches the inbox at all rather than the spam folder. Without solid deliverability, even the best subject line won't lift opens.