Written in response to something Hariprasad wrote.
That is good as $user passed to the function doesn't work when tried. I couldn't get much info from the codex as there is no documentation for it.
wpquestions should allow the question solver to post a reply if his answer has previously been accepted as a solution. So, in that manner, one can respond to any question the asker might have right after his answer has been accepted.
This'd eliminate the need for the send message and discourse feature.