Sends an SMS or MMS message to a single recipient. The sender number (from) must be an active number assigned to your account. For SMS, provide a body (max 640 characters). For MMS, provide a mediaUrl pointing to a publicly accessible media file; the media is downloaded, validated, and attached to the message. You may include both body and mediaUrl for MMS with text. An optional idempotencyKey prevents duplicate sends — if a message with the same key was already sent, the original response is returned.
Content compliance: Message content is validated against messaging compliance rules (SHAFT, phishing, etc.). Messages that violate these rules are rejected with error code 33019.
Rate limit: 10 requests per 60-second window. Every response includes X-RateLimit-Limit, X-RateLimit-Remaining, and X-RateLimit-Reset headers. When the limit is exceeded, a 429 response is returned with a Retry-After header.