A UAS that receives a MESSAGE request processes it following the
   rules of SIP [1].

   A UAS receiving a MESSAGE request SHOULD respond with a final
   response immediately.  Note, however, that the UAS is not obliged to
   display the message to the user either before or after responding
   with a 200 OK.  That is, a 200 OK response does not necessarily mean
   the user has read the message.

   A 2xx response to a MESSAGE request MUST NOT contain a body.  A UAS
   MUST NOT insert a Contact header field into a 2xx response.

   A UAS which is, in fact, a message relay, storing the message and
   forwarding it later on, or forwarding it into a non-SIP domain,
   SHOULD return a 202 (Accepted) [5] response indicating that the
   message was accepted, but end to end delivery has not been
   guaranteed.

   A 4xx or 5xx response indicates that the message was not delivered
   successfully.  A 6xx response means it was delivered successfully,
   but refused.