How can I solve this error?

See more:

C#

C#4.0

C#5.0

C#6.0

Hi When I run this code I've this problem: (The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.)
So, what should I do to solve it?

What I have tried:

protected void btPassRec_Click(object sender, EventArgs e)
    {
        String CS = ConfigurationManager.ConnectionStrings["MyDatabaseConnectionString1"].ConnectionString;
        using (SqlConnection con = new SqlConnection(CS))
        {
            SqlCommand cmd = new SqlCommand("select * from users where Email='"+tbEmailId.Text+"'",con);
            con.Open();
            SqlDataAdapter sda = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            sda.Fill(dt);

            if (dt.Rows.Count != 0)
            {
                String myGUID = Guid.NewGuid().ToString();
                int Uid = Convert.ToInt32(dt.Rows[0][0]);
                SqlCommand cmd1 = new SqlCommand("insert into ForgotPassRequests values('"+myGUID+"','"+Uid+"',getdate())",con);
                cmd1.ExecuteNonQuery();

                //send email
                String ToEmailAddress = dt.Rows[0][3].ToString();
                String Username = dt.Rows[0][1].ToString();
                String EmailBody = "Hi "+Username+ ",<br /><br /> Click the link below to reset your password <br /><br /> http://localhost:48599/RecoverPassword.aspx?Uid="+myGUID;
                MailMessage PassRecMail = new MailMessage("youremail@gmail.com", ToEmailAddress);
                PassRecMail.Body = EmailBody;
                PassRecMail.IsBodyHtml = true;
                PassRecMail.Subject = "Reset Password";

                //SmtpClient SMTP = new SmtpClient("smtp-mail.outlook.com", 587);
                SmtpClient SMTP = new SmtpClient("smtp.gmail.com", 587);
                SMTP.Credentials = new NetworkCredential()
                {
                    UserName = "youremail@gmail.com",
                    Password = "youremailpassword"      //https://www.youtube.com/watch?v=i6kXgf2MBl8
                };
                SMTP.EnableSsl = true;
                SMTP.Send(PassRecMail);

                lblPassRec.Text = "Check your email to reset your password.";
                lblPassRec.ForeColor = Color.Green;
                
            }
            else
            {
                lblPassRec.Text = "OOps This email id DOES NOT exist in our database !";
                lblPassRec.ForeColor = Color.Red;
            }
        }