As per notice from Google's

// C# code
using (SmtpClient smtpClient = new SmtpClient(SmtpDomain, SmtpPortNumber))
{
    smtpClient.DeliveryMethod = SmtpDeliveryMethod;
    smtpClient.UseDefaultCredentials = UseDefaultCredentials;
    smtpClient.EnableSsl = true;
    smtpClient.Credentials = new NetworkCredential(SmtpUsername, appsPasswrod);

    MailMessage mailMessage = new MailMessage { /* ... */ };
    
    smtpClient.Send(mailMessage);
}