SMTP is not sending emails in gmail business account - smtp

Question

This code is working well in gmail personal account, but when I try to use gmail business account, it is not working and keeps giving an error. 5.5.1 Authentication Required.
void SendEmail()
{
DataTable data = GetData();
DataTable email_data = GetEmailData();
data.TableName = "Employee_Data";
using (XLWorkbook wb = new XLWorkbook())
{
wb.Worksheets.Add(data);
using (MemoryStream memoryStream = new MemoryStream())
{
wb.SaveAs(memoryStream);
byte[] bytes = memoryStream.ToArray();
memoryStream.Close();
String from = "seong#abcd.net";
for (int i = 0; i < email_data.Rows.Count; i++)
{
String to = email_data.Rows[i][0].ToString();
using (MailMessage mm = new MailMessage(from, to))
{
mm.Subject = "Employees Attachment";
mm.Body = "Employees Exported Attachment";
mm.Attachments.Add(new Attachment(new MemoryStream(bytes), "Employees.xlsx"));
mm.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.EnableSsl = true;
System.Net.NetworkCredential credentials = new System.Net.NetworkCredential();
credentials.UserName = "seong#abcd.net";
credentials.Password = "1234";
smtp.UseDefaultCredentials = true;
smtp.Credentials = credentials;
smtp.Port = 587;
smtp.Send(mm);
}
}
}
}
}

Answer 1

I solved this problem.
The account should not use 2nd verification in gmail if you want to use SMTP.
https://support.google.com/accounts/answer/1064203?hl=en&ref_topic=7189195
I can't control those things, I ask the administrator allow not to use 2nd verification.
So I can work with that account using SMTP.

Answer 2

Smtp method for Business Gsuite
require_once('class.phpmailer.php');
$mail = new PHPMailer(); // defaults to using php "mail()"
$mail->IsSMTP();
$mail->SMTPDebug = 0;
$mail->SMTPSecure = "tls";
$mail->SMTPAuth = true;
$mail->Port = 587;
$mail->Host = "smtp.gmail.com";
$mail->Username = "Enter the user ID"; // SMTP account username
// SMTP account password
$mail->Password = "Enter your password";
$mail->SetFrom('Enter the User ID', 'Subject');
$mail->AddReplyTo("Enter the User ID", "Subject");
$mail->AddAddress($to, "Name");
$mail->Subject = "Contact Enquiry";
$message = '';
$mail->MsgHTML($message);
if($mail->Send()){
$mail->ClearAddresses();
echo "Mail Sent. Thank you " . $first_name . ", we will contact you shortly.";
} else {
echo "Mailer Error: " . $mail->ErrorInfo;
}