asp.net button disable postback but keep validation

Question

I want to make a password recovery function where the users needs to enter the mail adress. The mail adress has to be validated by asp.net. I dont want the site to refresh after reseting password. Thats why I disabled autopostback for my button. The problem is that disabling the autopostback also stopped the validation of the input. I dont want to use javascript to validate the input.

So is it possible to disable autopostback but keep the validation behaviour?

Thanks alot!

This is my asp.net code:

<asp:Label>Email: </asp:Label><asp:TextBox id="emailTextbox" TextMode="Email" runat="server" CSSClass="emailTextbox"/>
<asp:Button Text="Anfordern" runat="server" CSSClass="ResetPassword" CausesValidation="True"/>

and this is the called ajax behind the button

function ResetPassword() {
        alert($("#<%=emailTextbox.ClientID%>")[0].value);
        $.ajax({
            type: "POST",
            url: "/pages/passwordRecovery.aspx/ResetPassword",
            data: '{mail: "' + $("#<%=emailTextbox.ClientID%>")[0].value + '" }',
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: OnSuccess,
            failure: function (response) {
               //alert(response.d);
            }
        });
    }