Symptom: open rates spike across campaigns while clicks stay flat.
Cause: privacy-prefetching clients open (or pre-load) mail on the recipient's behalf. These are machine opens, not human attention.
Confirmation: check the sg_machine_open property on Opened events. A surge of machine opens matching the open-rate spike confirms it.
Fix:
1. Segment reporting: track human opens (machine open false) separately from total opens.
2. Stop making send decisions on raw open rate alone: sunset rules, A/B winners, and engagement scoring should weight clicks and replies, not prefetchable opens.
3. Do not 'fix' it by disabling open tracking; you lose the signal instead of cleaning it.
Verification: human-open trend stays continuous across the spike while total opens show the step change. Decisions made on the cleaned metric stop flip-flopping.