Cold email glossary
Open rate
Open rate is the percentage of delivered emails recorded as opened, measured by an invisible tracking pixel that loads when the message is displayed. Since Apple Mail Privacy Protection began preloading remote images in 2021, the metric can significantly overstate how many humans actually read a message.
What is open rate?
Open tracking works by embedding a tiny, invisible image in the email, hosted on the sender's tracking server. When the recipient's mail client loads that image, an open is logged. The rate is calculated as recorded opens divided by delivered messages, with bounces excluded from the denominator. A plain-text email with no pixel cannot be tracked for opens at all.
The measurement broke in both directions. Apple Mail Privacy Protection, introduced in 2021, routes remote images through Apple's proxy servers and preloads them whether or not the recipient ever looks at the message, so deliveries to Apple Mail can register as opens nobody made. Corporate security gateways often fetch images while scanning inbound mail, with the same effect. Meanwhile, clients that block remote images by default undercount real reads. The number you see is a blend of false opens and missed ones.
In cold email there is a second problem: the pixel itself can work against deliverability. The tracking image lives on a tracking domain, often one shared with many other senders, and spam filters recognize those domains. An HTML message carrying a tracking pixel also looks less like the personal, one-to-one mail that cold outreach is trying to resemble. For both reasons, many cold email operators now send plain-text style messages with open tracking turned off.
What replaced opens is the set of signals a sender can observe directly: replies, positive replies, bounces, and spam complaints. None of them can be inflated by a proxy server fetching an image.
Why it matters in cold email
Open rate still has one legitimate use in cold email: as a crude, campaign-level deliverability canary. If reported opens collapse across an entire batch, something probably changed about inbox placement. Beyond that, optimizing on opens tends to produce clickbait subject lines that win the open and lose the reply, while the tracking needed to measure them quietly works against placement. The goal of a cold email is a response, so the honest scoreboard is replies.
How Sendful handles it
Sendful judges campaigns on replies and positive replies, the signals that map to pipeline, instead of open numbers that modern mail clients have made unreliable. Weekly reporting shows those reply metrics alongside the deliverability signals that protect them.
What is a good open rate for cold email?
There is no benchmark worth trusting anymore. Since Apple Mail Privacy Protection and similar image prefetching, reported opens are inflated by machine loads, and the inflation varies with how many recipients use Apple Mail or sit behind security gateways. A sudden collapse in opens across a campaign can flag a deliverability problem, but as a performance metric, replies are the number to benchmark.
Does open tracking hurt deliverability?
It can. The tracking pixel is an image hosted on a tracking domain, and shared tracking domains accumulate reputation from every sender using them, including bad ones. Filters also treat HTML messages with remote images differently from plain text. Many cold email senders disable open tracking entirely and accept losing a metric that was already unreliable.
Why is my open rate high but my reply rate low?
Often because many of those opens were never human. Apple Mail preloads tracking pixels and corporate scanners fetch images during inspection, so a high open rate can coexist with very few real reads. If real people are reading and not replying, the issue is usually targeting or the offer, not the subject line.
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