2018: iMessage app messages can only be sent over iMessage

iMessage app messages can only be sent over iMessage. The request included an imessage_app part while explicitly requesting SMS or RCS. This is an HTTP 409 error.

• Omit preferred_service — it defaults to iMessage for app parts — or set it to iMessage
• iMessage app parts never fall back to SMS/RCS. If the recipient isn’t reachable over iMessage, the send instead fails asynchronously with a message.failed webhook carrying code 4005
• Check iMessage capability before sending

• iMessage Apps
• Protocol Selection
• Error Codes