E_mail 邮箱发送接受问题！

Question

报错误：邮箱不可用。 服务器响应为: 5.7.1 Unable to relay for [e-mail]

代码段：//发送邮件
            SmtpClient sc = new SmtpClient();
            sc.Host = "192.168.1.101";
            sc.Credentials = new NetworkCredential("[e-mail]", "begin2010-cpj");
            sc.Send(mailObj);
            Lab.Text = "发送完毕!";

急需解决！！！