用C#从POP3服务器取得邮件

Introduction

in this article, I'll show you how to retrieve mail from POP server based on RFC 1725.

Algorithm for Retrieving Mail

to retrieve mail from POP server, I just follow rule of RFC 1725. You also can download that paper (RFC 1725). 

here's the algorithm:   

 Client   : +OK Server POP  Ready!!
 Client   : USER xxx
 Server   : +OK
 Client   : PASS yyy
 Server   : +OK user successfully logged on
 Client   : STAT
 Server   : +OK n m
 Client   : RETR 1
 Server   : +OK
               ---{ data }-----
 Client   : QUIT
 Server   : +OK Server POP signing off

Implementation

it's easy to implement an application if we know the algorithm to retrieve mail from a POP server. In this article, I use the TcpClient and NetworkStream classes. Firstly, declare public variables:

 public TcpClient Server;
 public NetworkStream NetStrm;
 public StreamReader  RdStrm;
 public string Data;
 public byte[] szData;
 public string CRLF = "\r\n";

here's the code for when the Connect Button is clicked:

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

    // create server POP3 with port 110
    Server = new TcpClient(POPServ.Text,110);
    Status.Items.Clear();

    try
    {
        // initialization
        NetStrm = Server.GetStream();
        RdStrm= new StreamReader(Server.GetStream());
        Status.Items.Add(RdStrm.ReadLine());

        // Login Process
        Data = "USER "+ User.Text+CRLF;
        szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());
        NetStrm.Write(szData,0,szData.Length);
        Status.Items.Add(RdStrm.ReadLine());

        Data = "PASS "+ Passw.Text+CRLF;
        szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());
        NetStrm.Write(szData,0,szData.Length);
        Status.Items.Add(RdStrm.ReadLine());

        // Send STAT command to get information ie: number of mail and size
        Data = "STAT"+CRLF;
        szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());
        NetStrm.Write(szData,0,szData.Length);
        Status.Items.Add(RdStrm.ReadLine());

        // change enabled - disabled button
        ConnectBtn.Enabled = false;
        DisconnectBtn.Enabled = true;
        RetrieveBtn.Enabled = true;

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

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

here's the code for when the Disconnect Button is clicked:

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

    // Send QUIT command to close session from POP server
    Data = "QUIT"+CRLF;
    szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());
    NetStrm.Write(szData,0,szData.Length);
    Status.Items.Add(RdStrm.ReadLine());

    //close connection
    NetStrm.Close();
    RdStrm.Close();

    // change enabled - disabled button
    ConnectBtn.Enabled = true;
    DisconnectBtn.Enabled = false;
    RetrieveBtn.Enabled = false;

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

here's code when Retrieve Button clicked:

private void RetrieveBtn_Click(object sender, System.EventArgs e)
{
    // change cursor into wait cursor
    Cursor cr = Cursor.Current;
    Cursor.Current = Cursors.WaitCursor;
    string szTemp;
    Message.Clear();
    try
    {
        // retrieve mail with number mail parameter
        Data = "RETR "+ Number.Text+CRLF;
        szData = System.Text.Encoding.ASCII.GetBytes(Data.ToCharArray());
        NetStrm.Write(szData,0,szData.Length);

        szTemp = RdStrm.ReadLine();
        if(szTemp[0]!='-') 
        {
        
            while(szTemp!=".")
            {
                Message.Text += szTemp;
                szTemp = RdStrm.ReadLine();
            }
        }
        else
        {
            Status.Items.Add(szTemp);
        }
        
        // back to normal cursor
        Cursor.Current = cr;

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

}

Testing

build and run this project. Set the POP server, user and password. After that you'll get a response message from the POP server (you can see on status box) ie: +OK 2 624 if success or -ERR if fail. The words "+OK 2 624" mean you have two emails and total size 624.

now, you can set the mail message number and then click Retrieve button. Then you'll get the mail according to the mail number that you wrote on Mail Number Box.

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