身份验证失败时 MailKit 日志为空

问：

以下代码在处理后失败并出现错误：AuthenticateAsync()

MailKit.Security.AuthenticationException: 535: Authentication failed.
Restarting authentication process. ---> MailKit.Net.Smtp.SmtpCommandException:
Authentication failed. Restarting authentication process.

public async Task SendMailMessage(M.MailMessage message)
{
  using MemoryStream stream = new MemoryStream(10_240);
  using StreamReader reader = new StreamReader(stream);
  using ProtocolLogger logger = new ProtocolLogger(stream);
  using SmtpClient smtp = new SmtpClient(logger);

  try
  {
    await smtp.ConnectAsync(
      _config.HostName,
      _config.Port,
      _config.SecurityType == EMailSecurityType.StartTls ? SecureSocketOptions.StartTlsWhenAvailable : SecureSocketOptions.Auto
      );
    await smtp.AuthenticateAsync(_config.UserName, _config.Password);
    await smtp.SendAsync(new MimeMessage(message));
    await smtp.DisconnectAsync(true);
  }
  catch
  {
    stream.Flush();
    Debug.Print(reader.ReadToEnd());
  }
}

为什么日志是空的？