System.Net.Mail.SmtpException: 'Failure sending mail.'

Question

I am trying to make a simple website but on reset password page after typing any mail address and clicking on reset password button I am getting a error saying

System.Net.Mail.SmtpException: 'Failure sending mail.'

Inner Exception
IOException: Unable to read data from the transport connection: net_io_connectionclosed.

Here is my code

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

namespace ARTIst_Sketch_Work
{
public partial class ForgotPassword : System.Web.UI.Page
{

    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void Button1_Click(object sender, EventArgs e)
    {

        string Username = string.Empty;
        string Password = string.Empty;
        string constr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand("SELECT Username, Password FROM [Table] WHERE [E-mail] = @Email"))
            {
                cmd.Parameters.AddWithValue("@Email", TextBoxmail.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("sender@gmail.com", TextBoxmail.Text.Trim())
            {
                Subject = "Password Recovery",
                Body = string.Format("Hi {0},<br /><br />Your password is {1}.<br /><br />Thank You.", Username, Password),
                IsBodyHtml = true
            };
            SmtpClient smtp = new SmtpClient
            {
                Host = "smtp.gmail.com",
                EnableSsl = true
            };
            NetworkCredential NetworkCred = new NetworkCredential
            {
                UserName = "sender@gmail.com",
                Password = "<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.";
        }

    }
}

}

The exception pops at :

smtp.Send(mm);

Any help will be appreciated

Answer 1

Found a key solution.

If you are using .NET version 4.x, SMTP defaults to TLS 1.0. Most modern email systems (Comcast, Gmail, etc.) require you to use TLS 1.2

Simply add this code in the initial (Main) or top block

Imports System.Net.ServicePointManager
..
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 

More details for the C# version

https://learn.microsoft.com/en-us/answers/questions/400152/authentication-failed-because-the-remote-party-has

Answer 2

I suggest using MailKit.

https://learn.microsoft.com/en-us/dotnet/api/system.net.mail.smtpclient

Important under Remarks

We don't recommend that you use the SmtpClient class for new development because SmtpClient doesn't support many modern protocols. Use MailKit or other libraries instead. For more information, see SmtpClient shouldn't be used on GitHub.