Alert Box Problem

Question

hi!!
in my contact form, i am sending an email..what i want now is an alert box after email is sent sucessfully..
i am using the following code at the end of my email code::
Page.ClientScript.RegisterStartupScript(this.GetType(), "MyScript","alert('Email sent.Thanks for your response');", true);

its working fine.

BUT THE PROBLEM is that the alert box is not showing above the same page..but on the new/blank page with nothing on the page ..and when i click the "ok" button of the alert box then i get to see the contact page again..
is this any kind of error???

can anyone pls tell the problem and solve it or tell me the other possible way to to do it..
thanks!!