.NET 2.0中发送邮件的问题

写了一个发送邮件的类：

public class SendMail
{
private static string m_UserName;
private static string m_Password;
private static string m_HostName;

static string returnMSG = "";

public static void NewMailServer(string user, string pwd, string host){
m_UserName = user;
m_Password = pwd;
m_HostName = host;
}

public static string Send(string from, string sentto, string cc, string
bc, string subject, string body)
{
try
{
MailMessage message = new MailMessage(new MailAddress(from, "Rui " +
(char)0xD8 + " Chen", System.Text.Encoding.UTF8), new MailAddress(sentto));

message.Subject = subject;
message.Body = body;
message.IsBodyHtml = true;

SmtpClient mailclient = new SmtpClient();

mailclient.Host = m_HostName;
mailclient.Credentials = new NetworkCredential(m_UserName, m_Password);
mailclient.UseDefaultCredentials = false;
mailclient.Timeout = 100000;
mailclient.DeliveryMethod = SmtpDeliveryMethod.Network;

mailclient.Send(message);
return returnMSG;
}
catch (System.Net.Mail.SmtpException sEx)
{
return sEx.StatusCode.ToString() + " : " + sEx.Source + sEx.ToString();
}
}

private SendMail()
{

}
}

但是在调用的时候出现错误：

MailboxNameNotAllowed : System System.Net.Mail.SmtpException: Mailbox name
not allowed. The server response was: You are not authorized to send mail as
>, authentication is required at
System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String
response) at System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[]
command, String from) at System.Net.Mail.SmtpTransport.SendMail(MailAddress
from, MailAddressCollection recipients, SmtpFailedRecipientException&
exception) at System.Net.Mail.SmtpClient.Send(MailMessage message) at
Merlion.Web.Mail.SendMail.Send(String from, String sentto, String cc, String
bc, String subject, String body) in c:\Documents and Settings\陈锐\My
Documents\Visual Studio 2005\WebSites\WebSite3\App_Code\SendMail.cs:line 53

8直到是什么原因，搞了一下午，郁闷。不行干脆用Socket写一个自己的SMTP实现算了。