<form id="form1" runat="server">
    <div class="form">

<div style="padding-bottom:7px;"><input type="text" id="name" runat="server" class="textbox" value="Your full name"/></div>
        <asp:RequiredFieldValidator runat="server" ID="ValidateName" ControlToValidate="name"
         ErrorMessage="Имя пустое" Display="dynamic">
</asp:RequiredFieldValidator>

<div><asp:TextBox ID="TextBox3" runat="server" CssClass="textbox">Your email</asp:TextBox></div>
        <asp:RegularExpressionValidator runat="server" ID="ValidateEmail"
       ControlToValidate="TextBox3" ValidationExpression=".*@.{2,}\..{2,}"
       ErrorMessage="Некорректный формат E-mail" Display="dynamic">
</asp:RegularExpressionValidator>

 <div style="margin-top:20px;margin-left:49px; margin-bottom:15px;"> <button type="submit" id="bt" runat="server" class="button" ></button></div>

    </div>
</form>