از Gmail استفاده کردید برای ارسال ؟

بله

باید دسترسی را آزاد کنید

لینک های زیر را دنبال کنید

https://www.google.com/settings/security/lesssecureapps

2 --

http://www.smarterasp.net/support/kb/a1546/send-email-from-gmail-with-smtp-authentication-but-got-5_5_1-authentication-required-error.aspx

جناب مهندس مواردی رو که فرمودید انجام دادم ولی خطای جدید میده

The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at 

جناب مهندس کدی رو که استفاده کردم

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Net;
using System.Net.Mail;
using System.Drawing;
using System.Configuration;
using System.Data.SqlClient;

public partial class ForgotPasswordCS : System.Web.UI.Page
{
    protected void SendEmail(object sender, EventArgs e)
    {
        string username = string.Empty;
        string password = string.Empty;
        string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand("SELECT Username, [Password] FROM Users WHERE Email = @Email"))
            {
                cmd.Parameters.AddWithValue("@Email", txtEmail.Text.Trim());
                cmd.Connection = con;
                con.Open();
                using (SqlDataReader sdr = cmd.ExecuteReader())
                {
                    if (sdr.Read())
                    {
                        username = sdr["Username"].ToString();
                        password = sdr["Password"].ToString();
                    }
                }
                con.Close();
            }
        }
        if (!string.IsNullOrEmpty(password))
        {
            MailMessage mm = new MailMessage("*****@gmail.com", txtEmail.Text.Trim());
            mm.Subject = "Password Recovery";
            mm.Body = string.Format("Hi {0},<br /><br />Your password is {1}.<br /><br />Thank You.", username, password);
            mm.IsBodyHtml = true;
            SmtpClient smtp = new SmtpClient();
            smtp.Host = "smtp.gmail.com";
            smtp.EnableSsl = true;
            NetworkCredential NetworkCred = new NetworkCredential();
            NetworkCred.UserName = "";
            NetworkCred.Password = "<>";
            smtp.UseDefaultCredentials = true;
            smtp.Credentials = NetworkCred;
            smtp.Port = 587;
            smtp.Send(mm);
            lblMessage.ForeColor = Color.Green;
            lblMessage.Text = "Password has been sent to your email address.";
        }
        else
        {
            lblMessage.ForeColor = Color.Red;
            lblMessage.Text = "This email address does not match our records.";
        }
    }
}

این خطا 5.1 دقیقا به همون لینک هایی که دادم مربوط میشه

حتما درست انجام ندادید

متن خطا میگه کاملا گویاست