You need to create and Auto reply agent.
1. Goto Create ->Agent
2. In the properties box, enter any name for the agent and select ” After Mail Arrives” in the first drop down in runtime section.
3. Close the properties box and click on Add action button at the bottom of the window.
4. Select Reply to Send from the Action drop down menu..type ur message…save the agent….and exit
(This has to done on the server copy of the mail database and not on the local copy….otherwise response would be sent after certian delay…)