Queues an outbound message for sending via the RCS channel. The agent must exist and not be suspended, and the caller must have permission to send messages for the agent.

Two optional correlation fields behave differently — note the asymmetry:

• correlateId (max 128 chars) tracks outbound campaigns through delivery reports and inbound replies. It is not sticky: omitting it on a subsequent send clears any existing value on the conversation.
• conversationMetadata (JSON object, max 2048 chars serialized) attaches business context that persists on the conversation and is echoed back on inbound replies. It is sticky: omitting it does not clear an existing value.

Both are returned on the matching EndUserWebhookPayload callbacks as correlateId and conversationMetadata.