How to send correctly an email using System.Net.Mail

I was working in Visual Studio with Windows Forms. I added a method SendVerificationCode, to send the verification code using System.Net.Mail:

using System.Net.Mail;

namespace PasswordManager
{
    public partial class Form1 : Form
    {
        Random rnd = new();

        private void SendVerificationCode(object sender, EventArgs e)
        {
            if (!(textBox4.Text == "") && !(textBox2.Text == ""))
            {
                verificationCode = "";

                for (int _ = 0; _ < 4; _++)
                {
                    verificationCode += nums[rnd.Next(0, 10)];
                }

                try
                {
                    MailMessage mail = new MailMessage();
                    SmtpClient smtp = new SmtpClient();

                    mail.From = new MailAddress("**********@gmail.com");
                    mail.To.Add(textBox2.Text);
                    mail.Subject = "Verification code";
                    mail.Body = verificationCode;

                    smtp.Host = "smtp.gmail.com";
                    smtp.Port = 587;
                    smtp.Credentials = new System.Net.NetworkCredential("*********@gmail.com", "**********");
                    smtp.EnableSsl = true;

                    smtp.Send(mail);
                }
                catch (Exception)
                { 
                    throw new Exception("It's not working"); 
                }
            }
        }
    }
}

The textBox2.Text was ********@wp.pl.

It throws an exception "it’s not working".

Can somebody help me?

Thank you.

>Solution :

You’re trying to send from a Gmail address. Gmail no longer allows simple username/password authentication. Until recently you could enable "Less Secure Apps" and get a per-app password, but this is no longer available^*. Instead you’ll need to get an OAuth2 token… which is not supported in System.Net.Mail.

That is, you cannot do this with .NET’s built-in SMTP client.

Most people doing serious work have their own SMTP service. But for Gmail, there are third party libraries that can help. MailKit is one example.

_{* Technically this still available for some types of account, but it shuts off for good on Monday, less than a week from now.}