i have tried all of the combinations:
//MessageBox.Show("Email address saved.");
ScriptManager.RegisterStartupScript(this, GetType(), "showAlert", "alert('Email address saved.');", true);
//ScriptManager.RegisterStartupScript(this, GetType(), "displayalertmessage", "Showalert();", true);
//ScriptManager.RegisterStartupScript(this, this.GetType(), "showalert", "alert('Email address saved.');", true);
//this.Page.ClientScript.RegisterStartupScript(this.GetType(), Guid.NewGuid().ToString() , "alert('Email address saved.');__doPostBack('__Page', 'EmptyPostback');", true);
//Page.ClientScript.RegisterStartupScript(this.GetType(), "myalert", "alert('Email address saved.');", true);