validator callout extender for email validation

hi,
I use validator callout extender for email validation
If I full wrong value display error but again I put the correct value error message not close
My code, how to solve this problem

 <td class="Error">
            <asp:RegularExpressionValidator ID="REVEmail" runat="server" ControlToValidate="txtEmail"  ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" ErrorMessage="Enter Correct Formet" Display="None" ></asp:RegularExpressionValidator>
            <asp:ValidatorCalloutExtender ID="REVEmail_ValidatorCalloutExtender" runat="server" Enabled="True" TargetControlID="REVEmail" >
            </asp:ValidatorCalloutExtender>
        </td>

Regards
Sekar.c

Regards
Sekar.c