How does inactivity measurement differ across ESPs?

Ask two ESPs whether a subscriber is "inactive" and you might get two very different answers about the same person. That's not a bug. It's a feature of how each platform is built.

The main differences come down to what counts as an "open" and over what time window. Some ESPs count any pixel load as an open, including prefetched images from Apple Mail Privacy Protection. Others filter those out and only count opens they're confident are human. Since MPP fires a pixel for virtually every Apple Mail user, an ESP that counts those will show higher engagement and longer active windows than one that doesn't.

Click data is more consistent across platforms, but not entirely. Some ESPs count a click on any link, including tracking redirects that bot scanners hit. Others flag bot-like click patterns (millisecond timing, data center IPs) and exclude them from engagement metrics.

Klaviyo has built its own engagement scoring that weighs click behavior more than opens. Mailchimp defines engagement mostly through raw open and click events within a rolling window. Customer.io lets you define custom engagement events. A subscriber flagged as "at risk" in one platform might look perfectly healthy in another.

This matters most during ESP migrations. Don't export your "active" segment from the old platform and import it as-is. What each system calls active doesn't always match. Re-confirm engagement before migrating, or you'll carry a gap between your perceived and real active audience.

The safest cross-platform measure is click behavior. Clicks are harder to fake and less distorted by proxy caching than opens. If you're setting re-engagement thresholds or cleaning your list, weight clicks over opens when the two disagree.

Before an ESP migration, a list validation pass is worth the effort. Starts the new relationship fresh.