After giving this a little thought I think we should now return a 202 to the user instead of a 204.
I think returning 204 may have been inappropriate before as it signifies that "the server has fulfilled the request but does not need to return an entity-body, and might want to return updated metainformation."  I think we should have simply returned a 200 before. 

I think returning a 202 is more appropriate here because we are queueing the message for processing and returning without verifying that the processing has been completed.  202 is used when "The request has been accepted for processing, but the processing has not been completed."