Check EmailID Already exists or not in asp.net

<script language="javascript" type="text/javascript">

    function checkuserEmail(txt) {
        var valuseremail = txt;
        if (valuseremail != "") {

            WebService.check_ContentEditor_userEmail(valuseremail, OnSucceeded, OnFailed);
        }

    }
    function OnSucceeded(result, userContext, methodName) {

        var userarray = result;
        document.getElementById('<%=lblresult.ClientID %>').innerHTML = userarray;
        if (userarray != "<font color='green'>EmaiID Available,Please proceed !</font>") {
            document.getElementById('<%=txtEmail.ClientID %>').value = "";
        }


    }
    function OnFailed(error, userContext, methodName) {

        alert("An Error Occured During Check Your EmailID. Please Try After Some Time!!!");
    }

</script>


<asp:TextBox ID="txtEmail" runat="server" CssClass="serachtext1" onblur="checkuserEmail(this.value);"></asp:TextBox>
                <asp:RequiredFieldValidator ID="RFV_txtEmail" runat="server" ControlToValidate="txtEmail"
                    Display="None" SetFocusOnError="true" ErrorMessage="Enter EmailID" ValidationGroup="Insert"></asp:RequiredFieldValidator>
                <ajaxToolkit:ValidatorCalloutExtender ID="VCE3_txtEmail" CssClass="customCalloutStyle"
                    runat="server" WarningIconImageUrl="~/images/ajaxerror.png" CloseImageUrl="~/images/ajaxclose.png"
                    TargetControlID="RFV_txtEmail">
                </ajaxToolkit:ValidatorCalloutExtender>
                <asp:RegularExpressionValidator ID="REV_txtEmail" runat="server" ControlToValidate="txtEmail"
                    Display="None" SetFocusOnError="true" ErrorMessage="Enter valid EmailID" ValidationGroup="Insert"
                    ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*"></asp:RegularExpressionValidator>
                <ajaxToolkit:ValidatorCalloutExtender ID="VCE2_txtEmail" CssClass="customCalloutStyle"
                    runat="server" WarningIconImageUrl="~/images/ajaxerror.png" CloseImageUrl="~/images/ajaxclose.png"
                    TargetControlID="REV_txtEmail">
                </ajaxToolkit:ValidatorCalloutExtender>
                <asp:Label ID="lblresult" runat="server" Font-Bold="False" ForeColor="Red" Text=""></asp:Label>

[WebMethod]

    public string check_ContentEditor_userEmail(string emailid)

    {

        try

        {

            BAL.TravelBAL objtravelbal = new BAL.TravelBAL();  

            DataSet ds = new DataSet();

            ds = objtravelbal.Get_CityContentEditor_ByEmail(emailid);

            string result = "";

            if (ds.Tables.Count > 0 && ds.Tables[0].Rows.Count > 0)

            {

                result = "<font color='red'>EmaiID Already Exist,Please Enter Another EmailID!</font>";

            }

            else

            {

                result = "<font color='green'>EmaiID Available,Please proceed !</font>";

            }

            return result;

        }

        catch (Exception ex)

        {

            string result = "Error occur during checking your EmaiID";

            return result;

        }

    }