This feature is currently only available in the Core SDK and Unity SDK v16.
When you send a message to an offline user you will receive a
evt_account_send_message_failed
event. This can be used to relay the fact that the user receiving said message is offline. The message gets archived and when the user that received that message signs in, they can retrieve their account chat history to see the message that was sent to them while offline.
Offline direct messages • Vivox Core SDK • Unity Docs