4001: Delivery failed

Message could not be delivered. This may be due to an unreachable recipient or service disruption. This is an HTTP 500 error.

When reason begins with “Message delivery failed” (the full string may be “Message delivery failed. Please retry the message.”), a message.delivered webhook for the same message ID may still arrive later in some cases — as opposed to “Message send failed”, which is terminal. Send timeouts, which previously also surfaced as 4001, are now reported as code 4006 with the same may-still-deliver caveat stated explicitly.

• Try sending the message again
• If the problem persists, contact support with the trace_id from the error response

• Sending messages
• Debugging