Symptom: 202 accepted, then dropped events citing a suppression reason, for addresses you believe are fine.
Cause candidates:
1. Bounce list: the address hard-bounced before. Correct behavior.
2. Unsubscribe list: the recipient opted out, possibly via a different product surface.
3. Spam report list: the recipient reported spam; also correct behavior.
4. Stale sync: your CRM re-adds suppressed addresses every night.
Confirmation: check Suppressions in the console for the address and the list it sits on.
Fix:
1. For genuinely fixed addresses (typo corrected, re-opted-in with proof), remove from the specific list and resend once.
2. Fix the sync so suppressed addresses are never re-imported as mailable.
3. Never bulk-clear suppression lists to improve 'deliverability'. It does the opposite.
Verification: the address sends normally after targeted removal, and the sync stops re-adding suppressed addresses.