What metrics indicate engagement decay?
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Engagement decay is gradual. It rarely shows up as a sudden drop. It creeps in: open rates drift down a percent or two per month, click-to-open rate quietly falls, and you find yourself wondering if your content is getting worse or if something else is going on. Usually it's both, but the root cause is often the audience, not the content.
Falling open rates (trend, not a single campaign)
A single low open rate might be a bad subject line or a timing issue. Three months of declining open rates is a signal. Adjust for Apple Mail Privacy Protection (MPP) inflating your numbers. If you can isolate non-Apple users and their open rate is falling, that's a real signal.
Declining click-to-open rate (CTOR)
CTOR falling below 5-8% consistently means people are opening but not finding the content compelling enough to click. This can mean content drift (you're writing for a different audience than you started with), or that you've accumulated a lot of passive openers who open out of habit but never engage beyond the first few seconds.
Growing inactive segment
Watch the percentage of your list that hasn't engaged in 90, 180, and 365 days. If these numbers are growing month over month, that's decay. A list that was 15% inactive six months ago and is 28% inactive now has a problem that will compound over time unless you act on it.
Silent unsubscribers and complaint uptick
A small rise in complaint rate often precedes a larger engagement decline. The most frustrated subscribers complain first. The merely disinterested just go quiet. If complaints are ticking up at the same time open rates are falling, you're likely in early-stage decay from audience misalignment.
Delivery errors increasing
Soft bounce rates creeping up alongside engagement decline can signal that your domain reputation is slipping. Mailbox providers start deferring or rate-limiting traffic from senders with poor engagement signals. Check Google Postmaster Tools for domain reputation trends if you're seeing this.
What to do when you spot it
Segment by engagement recency first. Run a re-engagement campaign on the 90-180 day inactive group. If they don't respond, suppress them. Don't wait for the data to get worse before acting. The inactive cohort's silence is actively dragging down your reputation for your engaged subscribers.
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