Symptom: dropped events with reason 'Invalid Address'.
Cause: the address is syntactically invalid (typo, missing domain, stray characters) or came from a bad import or form input.
Confirmation: the dropped event names the address. Eyeball it; the defect is usually obvious.
Fix:
1. Validate addresses at capture: syntax check in the form, and confirm the domain has an MX record for important flows.
2. Clean the source: fix the import or the form that produced the bad addresses.
3. For bulk lists, run a validation pass before sending rather than discovering bad addresses one drop at a time.
Verification: Invalid Address drops fall to zero on the next send to the cleaned list.