ASCX CODE:
<%@ Register assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" namespace="System.Web.UI" tagprefix="asp" %>
<asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate>

<h2>Contact Form</h2>        
<div>Name:*</div>
            <div><asp:TextBox ID="tb_Name" runat="server" Width="220px"></asp:TextBox><asp:RequiredFieldValidator
                ID="RequiredFieldValidator1" runat="server" ErrorMessage="!!" 
                    ControlToValidate="tb_Name" CssClass="ErrorMessage"></asp:RequiredFieldValidator></div>
<div>Company Name:</div>
            <div><asp:TextBox ID="tb_CompanyName" runat="server" Width="220px"></asp:TextBox></div>
<div>Email:*</div>

            <div><asp:TextBox ID="tb_Email" runat="server" Width="220px"></asp:TextBox><asp:RequiredFieldValidator
                ID="RequiredFieldValidator2" runat="server" ErrorMessage="!!" 
                    ControlToValidate="tb_Email" CssClass="ErrorMessage"></asp:RequiredFieldValidator></div>
                <div><asp:RegularExpressionValidator
                    ID="RegularExpressionValidator1" runat="server" 
                ErrorMessage="Invalid Email" ControlToValidate="tb_Email" 
                ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" 
                        CssClass="ErrorMessage" Display="Dynamic"></asp:RegularExpressionValidator></div>
<div>Phone:*</div>

            <div><asp:TextBox ID="tb_Phone" runat="server" Width="220px"></asp:TextBox><asp:RequiredFieldValidator
                ID="RequiredFieldValidator3" runat="server" ErrorMessage="!!" 
                    ControlToValidate="tb_Phone" CssClass="ErrorMessage"></asp:RequiredFieldValidator></div>
                <div>
            <asp:RegularExpressionValidator ID="RegularExpressionValidator2" runat="server" 
                ControlToValidate="tb_Phone" ErrorMessage="Invalid Phone" 
                ValidationExpression="((\(\d{3}\) ?)|(\d{3}-))?\d{3}-\d{4}" 
                        CssClass="ErrorMessage" Display="Dynamic"></asp:RegularExpressionValidator></div>
<div>Inquiry Type:*</div>
        <div>
            <asp:DropDownList ID="ddl" runat="server" Width="220px">
                <asp:ListItem></asp:ListItem>
                <asp:ListItem>Sales Inquiry</asp:ListItem>
                <asp:ListItem>Customer Service</asp:ListItem>
                <asp:ListItem>Technical Support</asp:ListItem>
            </asp:DropDownList>
                <asp:RequiredFieldValidator ID="RequiredFieldValidator4" runat="server" 
                ControlToValidate="ddl" ErrorMessage="!!" CssClass="ErrorMessage"></asp:RequiredFieldValidator></div>

<div>Question/Comment:*</div>
            <div>
                <asp:TextBox ID="tb_Comment" runat="server" TextMode="MultiLine" Height="64px" 
                Width="220px"></asp:TextBox>
                <asp:RequiredFieldValidator ID="RequiredFieldValidator5" runat="server" 
                    ControlToValidate="tb_Comment" CssClass="ErrorMessage" ErrorMessage="!!"></asp:RequiredFieldValidator>
    </div>
    <br />
            <div><asp:Button ID="btn_Send" runat="server" Text="Submit" 
                    onclick="btn_Send_Click" Width="100px" /> <asp:CheckBox
                    ID="cb_SendMe" runat="server" Text="Send me a copy" /></div>
            <div id="ContactMsg" runat="server"></div>

</ContentTemplate>
</asp:UpdatePanel>

<asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel1">
    <ProgressTemplate>
        <div>
            <img src="~/CMSWebParts/ContactUs/image/PleaseWait.gif" />Please wait...</div>
    </ProgressTemplate>
</asp:UpdateProgress>


ASCX.CS CODE:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using CMS.PortalControls;
using CMS.GlobalHelper;
using CMS.CMSHelper;
using System.IO;
using System.Net.Mail;
public partial class CMSWebParts_ContactUs_ContactUs : CMSAbstractWebPart
{
    protected void Page_Load(object sender, EventArgs e)
    {
	// this should ensure that the validation group is unique
	   String validationGroup = "vg"+this.clientID;
	   //since we're dynamically creating the validation group name, 
	   //we need to programmatically set the validation group on your validators
	   RequiredFieldValidator1.ValidationGroup = validationGroup;
	    RequiredFieldValidator2.ValidationGroup = validationGroup;
		 RequiredFieldValidator3.ValidationGroup = validationGroup;
		  RequiredFieldValidator4.ValidationGroup = validationGroup;
		   RequiredFieldValidator5.ValidationGroup = validationGroup;
		   //Set the validation group on your button
		 btn_Send.ValidationGroup = validationGroup;  
    }
    protected void btn_Send_Click(object sender, EventArgs e)
    {
	//You must always call Page.Validate() before checking if Page.Isvalid
	//In this instance to solve your problem you want to call Page.Validate("yourvalidationgroup");
		Page.Validate(((Button)sender).ValidationGroup); // cast the sender as a button and get the validation group name from it
        if (Page.IsValid)
        {
            string fileName = Server.MapPath("~/CMSWebParts/ContactUs/ContactUs.txt");
            string mailBody = File.ReadAllText(fileName);

            mailBody = mailBody.Replace("##Type##", ddl.SelectedValue);
            mailBody = mailBody.Replace("##Name##", tb_Name.Text);
            mailBody = mailBody.Replace("##Company##", tb_CompanyName.Text);
            mailBody = mailBody.Replace("##Email##", tb_Email.Text);
            mailBody = mailBody.Replace("##Phone##", tb_Phone.Text);
            mailBody = mailBody.Replace("##Comments##", tb_Comment.Text);

            MailMessage myMessage = new MailMessage();
            myMessage.Subject = "Inquiry message";
            myMessage.Body = mailBody;
            myMessage.From = new MailAddress(tb_Email.Text.Trim(), tb_Name.Text);
            myMessage.To.Add(new MailAddress("test@xxxx.com", "Steven"));
            if (cb_SendMe.Checked)
                myMessage.CC.Add(new MailAddress(tb_Email.Text.Trim(), tb_Name.Text));
            

            try
            {
                SmtpClient mySmtpClient = new SmtpClient();
                mySmtpClient.Send(myMessage);
                ddl.SelectedValue = "";
                tb_Name.Text = "";
                tb_CompanyName.Text = "";
                tb_Email.Text = "";
                tb_Phone.Text = "";
                tb_Comment.Text = "";
                cb_SendMe.Checked = false;
                ContactMsg.InnerHtml = "Your message has been sent.<br />Thank you!";
            }
            catch (Exception)
            {
                
                throw;
            }

        }
    }
}