SMTH-AUTH errors with System.Net.Mail

If you are getting SMTH AUTH errors while trying to send e-mails using the SmtpClient to an smtp server that requires authentication, make sure that you call UseDefaultCredentials before settings the credentials.

So, order is important here!

using System.Net;
using System.Net.Mail;
...
using (var client = new SmtpClient())
{
  client.Host = "smtp.teamfoundationserver.de";
  client.Port = 25;
  client.UseDefaultCredentials = false;
  client.Credentials = new NetworkCredential(username, password);
  client.Send(message);
}

Good luck!