用c#做邮箱发邮件失败出现smtpException 未处理什么意思

SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.163.com";

MailAddress Receive = new MailAddress("XXXXX@163.com");

MailAddress SendOut = new MailAddress("XXXXXX@163.com","XX");

MailMessage mail = new MailMessage(SendOut, Receive);

mail.Body = "Test";

mail.Subject = "Test";

NetworkCredential myCredentials = new NetworkCredential("XXXX@163.com", "********");
smtp.Credentials = myCredentials;

smtp.Send(mail);

Console.ReadKey();