不使用system.web.mail来发送邮件

Based on RFC 821 about Simple Mail Transfer Protocol, it's very easy to send mail via SMTP. Here's the handshaking for sending mail:
 
 Receiver: 220 server.com Simple Mail Transfer Service Ready
 Sender  : HELO server.com
 Receiver: 250 server.com
 Sender  : MAIL FROM: <[email protected]>
 Receiver: 250 OK
 Sender  : RCPT TO: <[email protected]>
 Receiver: 250 OK
 Sender  : DATA
 Receiver: 354 Start mail input: end with <CRLF>.<CRLF>
 Sender  : all data like From, To, Subject, Body etc.
           ended with <CRLF>.<CRLF>
 Receiver: 250 OK
 Sender  : QUIT
 Receiver: 250 server.com closing transmission channel

private void SendBtn_Click(object sender, System.EventArgs e)
{
    // change cursor into wait cursor
    Cursor cr = Cursor.Current;
    Cursor.Current = Cursors.WaitCursor;

    // create server SMTP with port 25
    TcpClient SmtpServ = new TcpClient(ServSMTP.Text,25);
    string Data;
    byte[] szData;
    string CRLF = "\r\n";
    LogList.Items.Clear();            

    try
    {
        // initialization
        NetworkStream NetStrm = SmtpServ.GetStream();
        StreamReader  RdStrm= new StreamReader(SmtpServ.GetStream());
        LogList.Items.Add(RdStrm.ReadLine());

        
        // say hello to server and send response into log report
        Data = "HELLO server " + CRLF;                
        szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());
        NetStrm.Write(szData,0,szData.Length);
        LogList.Items.Add(RdStrm.ReadLine());

        // send sender data
        Data = "MAIL FROM: " + " < " + sFrom .Text + " > " + CRLF;
        szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());
        NetStrm.Write(szData,0,szData.Length);
        LogList.Items.Add(RdStrm.ReadLine());

        // send receiver data
        Data = "RCPT TO: " + " < " + sTo .Text + " > " + CRLF;
        szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());
        NetStrm.Write(szData,0,szData.Length);
        LogList.Items.Add(RdStrm.ReadLine());

        // send DATA
        Data = "DATA " + CRLF;
        szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());
        NetStrm.Write(szData,0,szData.Length);
        LogList.Items.Add(RdStrm.ReadLine());                

        // send content data
        Data = "SUBJECT: " + sSubject.Text + CRLF + sMessage.Text + CRLF + "." + CRLF;
        szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());
        NetStrm.Write(szData,0,szData.Length);
        LogList.Items.Add(RdStrm.ReadLine());                

        // quit from server SMTP
        Data = "QUIT " + CRLF;
        szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());
        NetStrm.Write(szData,0,szData.Length);
        LogList.Items.Add(RdStrm.ReadLine());                

        // close connection
        NetStrm.Close();
        RdStrm.Close();
        LogList.Items.Add("Close connection");
        LogList.Items.Add("Send mail successly..");

        // back to normal cursor
        Cursor.Current = cr;

    }
    catch(InvalidOperationException err)
    {
        LogList.Items.Add("Error: "+ err.ToString());
    }


}