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client.Credentials = new NetworkCredential(sSMTPuser, sSMTPpass);
client.Credentials = new NetworkCredential(user@domainName.com, sSMTPpass);
用户名需要带上域名，真是脑残的邮件服务器： 亿邮。
login SMTP send mail error : Unable to read data from the transport connection: net_io_connectionclosed
原文地址：http://www.cnblogs.com/leavind/p/5643740.html
