A unified messaging system allows the receipt and sending of different messages across devices is established by creating relationships that leverage the capabilities of different devices. A message server establishes a relationship with a mobile device. Through the relationship, the message server can use the mobile device to send different types of messages that the server computer cannot transmit. A relationship between a client and a server extends this capability to the client. Through these relationships, a client can retrieve mobile device messages as well as generate them. The generated mobile device messages are transmitted to the message server. The message server determines that the received messages are mobile device messages that the server cannot deliver and sends the mobile device messages to the user's mobile device. The mobile device receives the mobile device messages from the message server and transmits the mobile device message to recipient mobile devices.