Symptom: sends return 429 with error type daily_quota_exceeded ("You have exceeded your daily email sending quota") or monthly_quota_exceeded.
1. Confirm the cause: this is a volume quota, distinct from the per-second rate limit. Check the x-resend-daily-quota and x-resend-monthly-quota response headers to see your usage.
2. Know the reset rules: the daily quota applies only to the Free plan. It runs on a UTC calendar day (00:00 to 24:00 UTC) and resets at midnight UTC. It is not a rolling 24-hour window, so "it has been 24 hours" does not clear it.
3. Count correctly: both sent and received emails count toward the quotas, so inbound mail consumes your budget too.
4. Fix: for a daily quota hit, wait until midnight UTC, or upgrade the plan to remove the daily quota. For a monthly quota hit, upgrade the plan to raise the monthly limit.
5. Verify: after reset or upgrade, resend and confirm a 200 with an email id. Set up monitoring on the quota headers so a scheduled job does not discover the limit at 2am.