163.com的郵箱在早期申請的是支持的，后面申請的都是不支持。
21cn.com似乎現在已經放棄了smtp支持，可能只有繳費用戶才可以。
qq.com的郵箱需要在用戶帳戶里面設置。

MailAddress from = new MailAddress("sosuo8@sina.com");
MailAddress to = new MailAddress("ahuinan@21cn.com");
MailMessage message = new MailMessage(from, to);
message.Subject = "阿會楠的來信";
message.Body = "你好！測試而已";
SmtpClient client = new SmtpClient();
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.Port = 25;
client.Host = "smtp.sina.com";
client.Credentials = new System.Net.NetworkCredential("sosuo8@sina.com", "123");
Response.Write("發送一份郵件到" + to.User + "," + to.Host +"," + client.Host);
client.Send(message);

using System.Net;
using System.Net.Mail ;