Symptom: dropped events with reason 'Unsubscribed Address'.
Cause: the recipient opted out (via your unsubscribe link, a group unsubscribe, or a spam complaint that became a suppression). SendGrid refuses to send to them.
Confirmation: the dropped event names the address and the reason. Cross-check your own unsubscribe records for the same address.
Fix:
1. Respect it. Remove the address from active mailing lists. Mailing unsubscribed addresses violates anti-spam rules and the doc's explicit opt-out requirement.
2. If the user re-subscribes through a confirmed opt-in, clear the suppression for that address first, then resume mailing.
3. Audit how the address got mailed: a list sync that ignores suppression state, or two systems (marketing + transactional) sharing one list.
Verification: after the list fix, dropped Unsubscribed Address events for that address stop. If they continue, the suppression was never cleared or the sync is still broken.