If your sends are failing, this is the order to debug:
1. Check the dashboard event log. Every failure has an event with a reason. 90% of issues are diagnosed here without further tooling.
2. Verify your sending domain is fully authenticated. Domains tab in the dashboard — DKIM, SPF, and tracking CNAMEs should all show green. A red status means delivery will be limited or blocked at the recipient.
3. Check suppressions. Suppressed addresses return success at API call time but produce no actual send. The dashboard's Suppressions tab shows global, per-group, and account-wide entries.
4. Check rate limits. Headers X-RateLimit-Remaining and X-RateLimit-Reset show your current state. Free tier is 600 req/sec per key.
5. Open a ticket. Email support@inbox.onesourcesoft.com with the message ID. We respond same-business-day on all paid plans.