Engagement window
The lookback period (typically 30-90 days) over which receivers measure subscriber engagement for placement decisions.
The engagement window is the time period over which mailbox providers measure recipient engagement when deciding placement. Gmail's window is roughly 30 days; Outlook's is shorter at 7-14 days; Yahoo's is around 30 days. Email from senders whose recent engagement is below threshold gets routed to spam or promotions, even if the long-term reputation is good.
This has operational consequences. A sender who pauses for 60 days will see deliverability degrade even when they resume normal sends — the recent-engagement signal has decayed. A sender who sends infrequently to a heavily-engaged list (active subscribers) outperforms a daily sender to a partially-engaged list, all else equal.
The practical takeaway is that engagement is a moving target. Maintaining good placement requires consistent send cadence to engaged subscribers, with proactive pruning of unengaged ones before they drag down the recent-engagement signal. "Suppress at 90 days inactive" is a typical policy that's tuned to maintain the engagement-window signal.