Don't reply. It is best if you are shy or not. Acknowledging someone who is rude just feeds their ego. By not replying you are saying they are not worth your time. The only exception to this would be if the person is your boss. In that case you keep it simple and to the point. Then you go to HR with the rude message in hand and file a complaint. If it is small company and the boss is the owner, you brush off your resume and look for another job.
