Interface for properties of received messages.
Allows the receiver to check whether this message has been cached on the server (Applies to peer-to-peer message only).
NoteThis method returns false if a message is not cached by the server. Only if the sender sends the message as an offline message (sets enableOfflineMessaging as true) when the specified user is offline, does the method return true when the user is back online.
true: This message has been cached on the server (the server caches this message and re-sends it to the receiver when he/she is back online).
false: This message has not been cached on the server.
The timestamp (ms) of when the messaging server receives this message.