用139邮箱测试成功，有office365邮箱测试报

msg.From = new MailAddress(from, fromname, System.Text.Encoding.UTF8);
/* 上面3个参数分别是发件人地址（可以随便写），发件人姓名，编码*/
msg.Subject = "这是测试邮件";//邮件标题
msg.SubjectEncoding = System.Text.Encoding.UTF8;//邮件标题编码
msg.Body = "邮件内容";//邮件内容
msg.BodyEncoding = System.Text.Encoding.UTF8;//邮件内容编码
msg.IsBodyHtml = true;//是否是HTML邮件
msg.Priority = MailPriority.High;//邮件优先级
SmtpClient client = new SmtpClient();
client.Credentials = new System.Net.NetworkCredential(username, password);
//上述写你的GMail邮箱和密码
client.Port = 587;//Gmail使用的端口
client.Host = server;
client.EnableSsl = true;//经过ssl加密
object userState = msg;
try
{
//client.SendAsync(msg, userState);
ServicePointManager.ServerCertificateValidationCallback = new RemoteCertificateValidationCallback(ValidateServerCertificate);
// client.SendAsync(msg, userState);
//简单一点儿可以
client.Send(msg);
return "send ok";
}
catch (System.Net.Mail.SmtpException ex)
{
return ex.Message;
}

if ((SslPolicyErrors.RemoteCertificateChainErrors & sslPolicyErrors) == SslPolicyErrors.RemoteCertificateChainErrors)
{
string msg = "";
foreach (X509ChainStatus status in chain.ChainStatus)
{
msg += "status code ={0} " + status.Status;  报NotSignatureValid
msg += "Status info = " + status.StatusInformation + " "; 报无法验证证书的签名
}
MessageBox.Show("证书链错误:"+ msg);  

最后仔细查了下原来是

SMTP 服务器要求安全连接或客户端未通过身份验证。 服务器响应为: 5.7.57 SMTP; Client was not authenticated to send anonymous mail during MAIL FROM

这个问题，请问怎么解决