Hi,
you can use ..
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:RegularExpressionValidator ID="regExEmail" runat="server" ControlToValidate="TextBox1" ErrorMessage="Please enter proper EMAIL id" ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+
[-.]\w+)*"></asp:RegularExpressionValidator>
you have to select RegularExpressionValidator from validation list..