How does frequency affect engagement decay?
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Send more often and your activity window for measuring inactivity gets shorter. That's the core relationship.
If you're mailing daily, someone who hasn't opened in 30 days has ignored 30 consecutive emails. That's a pretty clear signal. You'd reasonably call them inactive at 30 days.
If you're mailing monthly, someone who hasn't opened in 30 days has only had one chance. Calling them inactive at that point would be absurd. Monthly senders typically need 4 to 6 months of silence before inactivity is a meaningful signal.
Weekly senders usually land somewhere in between: 60 to 90 days is a reasonable threshold, giving subscribers 8 to 12 missed opportunities before you consider them gone.
Why this matters for deliverability
Inbox providers like Gmail measure engagement signals across your full sending volume. When a large portion of your list isn't engaging, it affects how your mail is treated for everyone, including your active readers. This is why sending to a huge, partially-disengaged list at high frequency often hurts more than it helps.
High sending frequency can also accelerate the decay of borderline subscribers. Someone who's mildly interested might happily read a monthly email but get overwhelmed by a daily one and start ignoring everything. Over time, you've turned a warm subscriber cold through volume.
Adjusting for frequency changes
If you've recently changed your sending frequency, update your inactivity thresholds accordingly. A subscriber who was "active" under a monthly sending cadence might not have had enough chances to click under a weekly one. Give them time before you flag them.
And if you're planning to increase frequency, consider running a re-engagement pass first. Remove the clearly disengaged before you start sending more. The active segment you're left with will respond better, and your deliverability won't take a hit from mailing a cold audience more often.
For a deeper look at how to set your thresholds, see the companion question on defining inactive subscribers.
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