Definition
Hard bounces are dead addresses that should suppress immediately. Accumulating them signals list decay and neglect to receiving servers, dragging every other campaign's placement down with them.
Why it matters
Deliverability is reputational arithmetic: bounce ratios above 2-3% start taxing inbox rates across the board.
Formula
(Hard bounces / Emails sent) x 100
Benchmarks
Keep under 2%; investigate spikes immediately
All glossary terms