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Start with the HTTP status and structured problem response. Then confirm the authenticated project, request shape, domain, recipient state, and current usage.

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Send-email responses

The send endpoint requires from, to, and subject plus at least one of html or text. An audience ID must be the only to value and cannot be combined with cc or bcc.

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Cancellation responses

A single-message cancellation succeeds only while the record is scheduled. If dispatch already began, Signal returns a conflict rather than claiming the message was cancelled. The bulk cancellation operation affects messages that are currently scheduled; it does not recall messages already handed off.

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Structured problems

Capture the response’s problem type, title, status, detail, and structured fields. Do not reduce every failure to the HTTP reason phrase; the problem body identifies the relevant customer condition.

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Before retrying

  • Correct 400, 401, 403, 413, and 422 conditions first.
  • For 429, honor Retry-After and use bounded backoff.
  • Preserve the same body idempotencyKey for the same logical send.
  • Do not automatically resend bounced, complained, unsubscribed, or suppressed recipients.
  • For indeterminate, inspect the existing record before creating any new send.

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Evidence for support

Provide the Signal email ID, UTC timestamp, endpoint, HTTP status, problem type, and project identifier. Redact API keys, Authorization headers, recipient content, and attachment data.

API errors

Review the shared problem format and status-code behavior.