Error response
{
"error": {
"code": "quota_exceeded",
"message": "Monthly recipient quota exhausted"
},
"request_id": "req-123"
}
400 invalid payload, 401 invalid credentials, 403 role/account/domain restriction, 404 inaccessible resource, 409 idempotency conflict, 422 suppressed/unsafe message, 429 quota/rate limit, 503 disabled or unconfigured service. Bodies and credentials never belong in application logs.
Domain still pending
Compare the exact record names and values with the current domain screen. DNS hosts may append the zone automatically. A selector from a previous domain claim will not verify a new claim. Keep mail records unproxied and allow DNS caches to refresh before rechecking. Read domain setup and use the DNS tools.
Accepted but not delivered
queued means accepted; submitted means accepted by the sending provider. Inspect the recipient outcomes and timeline for deferrals, bounces, suppression or warm-up pacing. needs_review requires operator reconciliation; do not resend blindly. Read delivery states.
Timeout after sending
A timeout can occur after acceptance. Retry the exact message with the same idempotency key, or inspect message history. A new key can create a duplicate. A 409 indicates the original key was reused with different content.
Webhook signature fails
Verify the exact raw request body before JSON parsing. Confirm the correct endpoint secret and inspect your server clock. The default timestamp tolerance is 300 seconds. During rotation, accept the signatures supplied by the verifier. See webhooks.
Cannot see a mailbox
Check the member's explicit mailbox assignment, including for owners and administrators. API keys inherit the issuing member's access; legacy ownerless keys cannot read private inboxes. A receiving domain alone does not enable replies: its name must also be a verified sending domain.
Gather useful diagnostics
Keep the request ID, timestamp, endpoint, HTTP status and error code. Do not log or share API keys, signing secrets, message bodies or recipient lists. Use the email's timeline and webhook delivery history to distinguish an API failure from a delivery problem.