CustomValidator OnServerValidate is not running

Hi All,

I have a textbox for email address, First I want to validate the format of the email address and the check from database that the given email address is available. Now at first I had a required field validator and regular expression validator and then for checking email address from database I am going to use Custom Validator. Now the real problem that I am facing is that OnServerValidate event is not running and I have no idea what is wrong with it. 

<tr><td><asp:TextBox ID="txtEmpNumber" CausesValidation="true" AutoPostBack="true"  runat="server" CssClass="standardTextBox" ValidationGroup="PageLevel"
                    MaxLength="50"></asp:TextBox><asp:CustomValidator ErrorMessage="errormessage" ValidationGroup="PageLevel" ControlToValidate="txtEmpNumber" OnServerValidate="EmailValidate" Text="Please enter a valid email address"
                                                            runat="server" /><asp:UpdatePanel ID="updatePanelCheckUserName" runat="server" UpdateMode="Conditional"><ContentTemplate><asp:Label ID="lblValidUserId" EnableViewState="false" runat="server" CssClass="ValidationError"></asp:Label></ContentTemplate><Triggers><asp:AsyncPostBackTrigger ControlID="txtEmpNumber" EventName="TextChanged" /></Triggers></asp:UpdatePanel></td><td><asp:FileUpload ID="FileUpload1" runat="server" /><br /><asp:RegularExpressionValidator EnableViewState="false" ID="regValidator1" runat="server"
            ControlToValidate="FileUpload1" ValidationExpression="[a-zA-Z0_9].*\b(.jpg|.JPG|.gif|.GIF|.jpeg|.JPEG|.png|.PNG)\b"
            ErrorMessage="Invalid image file. Please select a gif, jpeg, jpg or png file."
            EnableClientScript="true" Display="Dynamic" CssClass="ValidationError"></asp:RegularExpressionValidator></td></tr>

protected void EmailValidate(object source, ServerValidateEventArgs args)
{
       args.IsValid = false;
}

Any help would be greatly appriciated. 

Thanks,

Attiqe