What is dwell time in email?

You send an email, someone opens it, and your ESP logs how long that message stayed open. That duration is called dwell time. It sounds like a meaningful signal: a long dwell time suggests the reader actually read your email, while a short one might mean they glanced and moved on.

Most ESPs estimate dwell time by watching how long a tiny tracking pixel stays active in an open message. When the email closes, the pixel goes quiet, and the timer stops. Simple enough in theory.

The problem is that dwell time is now one of the least reliable metrics in your dashboard. Apple Mail Privacy Protection (MPP) changed everything when it launched in 2021. MPP pre-loads pixels on Apple's servers before the recipient even opens the email. Your ESP sees the pixel fire, starts a timer, and records a dwell time that has nothing to do with what the actual human did. The person may never have read a single word.

Apple Mail is used by a huge share of email clients worldwide, so this isn't a niche edge case. If a significant portion of your audience reads on Apple devices, your dwell time data is largely noise.

That doesn't mean you should ignore engagement metrics entirely. Clicks are still a much stronger signal because a proxy can't fake a deliberate link tap. Mailbox providers also track their own engagement signals that never show up in your ESP reports at all.

So treat dwell time as directional at best. When you see it alongside clicks, replies, and scroll data (where available), it can add texture. On its own, post-MPP, it's not something worth building strategy around.