Engagement
Recipient interactions with email — opens, clicks, replies, forwards. ISPs use engagement as the primary deliverability signal.
Engagement, in email, refers to recipient interactions with your messages: opens, clicks, replies, forwards, marking as "not spam," moving from Promotions to Primary. ISPs increasingly use engagement as the primary signal for sender reputation, far more than older signals like sending volume or content keywords.
The engagement signals weighted highest by ISPs are the high-effort ones: replies, forwards, moving to Primary, and marking as "not spam." Lower-effort signals (passive opens, especially since Apple Mail Privacy Protection) carry less weight. Designing for high-effort engagement (asking for replies, encouraging forwards) doesn't just look good — it directly improves deliverability.
The flip side: sending consistently to a low-engagement list degrades your reputation across all sends. The single most important list-hygiene practice is removing chronically disengaged subscribers. Counterintuitively, a smaller engaged list outperforms a larger disengaged one on both deliverability and conversion.