RegularExpressionValidator for Email:

<asp:RegularExpressionValidator ID="RegularExpressionValidator4" runat="server" ControlToValidate="txtEmail"
                                ForeColor="Red" ErrorMessage="EX:email@domainname.com" ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*"
                                Display="Dynamic" ValidationGroup="AttendeeValidationGroup"></asp:RegularExpressionValidator>