Failure sending mail

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C#

ASP.NET

I've create Forgot Password By Email Page Code In Asp.Net
this is HTML SOURCE

  <form id="Form1" runat="server">
  <div>
  <fieldset>
 <legend>Forgot Password</legend>
 <asp:Label ID="lblEmail" runat="server" Text="Email Address: "/>
 <asp:TextBox ID="txtEmail" runat="server"/>

  <asp:RequiredFieldValidator ID="RV1" runat="server"
                             ControlToValidate="txtEmail"
                             ErrorMessage="Please Enter EmailID"
                            SetFocusOnError="True">*
 </asp:RequiredFieldValidator>

 <asp:Button ID="btnPass" runat="server" Text="Submit"
                        onclick="btnPass_Click"/>

 <asp:ValidationSummary ID="ValidationSummary1"
                        runat="server" CssClass="error"/>

 <asp:Label ID="lblMessage" runat="server" Text=""/>
</fieldset>
</div>
 </form>

using System;

   using System.Data.SqlClient;

   using System.Configuration;

   using System.Data;

   using System.Net.Mail;



   protected void btnPass_Click(object sender, EventArgs e)

   {

       //Create Connection String And SQL Statement

       string strConnection = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;

       string strSelect = "SELECT UserName,Password FROM Users WHERE Email = @Email";



       SqlConnection connection = new SqlConnection(strConnection);

       SqlCommand command = new SqlCommand();

       command.Connection = connection;

       command.CommandType = CommandType.Text;

       command.CommandText = strSelect;



       SqlParameter email = new SqlParameter("@Email", SqlDbType.VarChar, 50);

       email.Value = txtEmail.Text.Trim().ToString();

       command.Parameters.Add(email);



       //Create Dataset to store results and DataAdapter to fill Dataset

       DataSet dsPwd = new DataSet();

       SqlDataAdapter dAdapter = new SqlDataAdapter(command);

       connection.Open();

       dAdapter.Fill(dsPwd);
       connection.Close();

       if(dsPwd.Tables[0].Rows.Count > 0 )

       {

           MailMessage loginInfo = new MailMessage();

           loginInfo.To.Add(txtEmail.Text.ToString());

           loginInfo.From = new MailAddress("YourID@gmail.com");

           loginInfo.Subject = "Forgot Password Information";



           loginInfo.Body = "Username: " + dsPwd.Tables[0].Rows[0]["UserName"] + "<br><br>Password: " + dsPwd.Tables[0].Rows[0]["Password"] + "<br><br>";

           loginInfo.IsBodyHtml = true;

           SmtpClient smtp = new SmtpClient();

           smtp.Host = "smtp.gmail.com";

           smtp.Port = 587;

           smtp.EnableSsl = true;

           smtp.Credentials = new System.Net.NetworkCredential("YourGmailID@gmail.com", "YourGmailPassword");

           smtp.Send(loginInfo);

           lblMessage.Text = "Password is sent to you email id,you can now <a href="Login.aspx">Login</a>";

       }

       else

       {

           lblMessage.Text = "Email Address Not Registered";

       }



   }

but when run that this error appear :
"Failure sending mail."
What i can do to solve this problem ?