The MessagingService contacts the remote centralized Messaging service to
retrieve any new messages. The message will still be kept on the remote
service after this retrieval. This method returns nothing, the
NetworkingService will inform the MessagingService when it receives the
reply from the remote service.
Sends a message to another user and also stores it in the Mailbox. The
message will be sent to the remote Messaging Service, which will store it
in the destination user's mailbox.