Between the calls to Connect and Disconnect, the client may call three other methods, List, Retrieve and Delete, any number of times. The client will usually begin by calling our List method to retrieve an array of messages that are queued on the POP3 server.

public ArrayList List()
{
    string message;
    string response;

    ArrayList retval = new ArrayList();
    message = "LIST\r\n";
    Write(message);
    response = Response();
    if (response.Substring(0, 3) != "+OK")
    {
        throw new Pop3Exception(response);
    }

    while (true)
    {
        response = Response();
        if (response == ".\r\n")
        {
            return retval;
        }
        else
        {
            Pop3Message msg = new Pop3Message();
            char[] seps = { ' ' };
            string[] values = response.Split(seps);
            msg.number = Int32.Parse(values[0]);
            msg.bytes = Int32.Parse(values[1]);
            msg.retrieved = false;
            retval.Add(msg);
            continue;
        }
    }
}

After sending the LIST message to the POP3 server, the server will respond with a +OK acknowledgement, followed by several lines representing one message each and finally by a line with a single period indicating the end of the messages. Each message line has two numbers, the first indicating the unique number of the message and the second indicating the message size in bytes.

Our List method will return a list of Pop3Message objects. The objects will only contain the message number and size of each message. In order to retrieve the full message, you can pass the message object to the Retrieve method. The Retrieve method will then respond with another Pop3Message containing the message content.

public Pop3Message Retrieve(Pop3Message rhs)
{
    string message;
    string response;

    Pop3Message msg = new Pop3Message();
    msg.bytes = rhs.bytes;
    msg.number = rhs.number;

    message = "RETR " + rhs.number + "\r\n";
    Write(message);
    response = Response();
    if (response.Substring(0, 3) != "+OK")
    {
        throw new Pop3Exception(response);
    }

    msg.retrieved = true;
    while (true)
    {
        response = Response();
        if (response == ".\r\n")
        {
            break;
        }
        else
        {
            msg.message += response;
        }
    }

    return msg;
}

To retrieve a message from a POP3 server, we send a RETR message with the unique message number. The server then responds with the +OK acknowledgement, the message content and finally the single period terminating line.