How to read POP3 emails from C #?

What I want to do, is read a type of attachment, for example those that are .pdf, of those emails that have not been read. To begin, I was trying with a simple example to read, but it tells me the following message:

Server did not accept user credentials

And I'm sure the username and password are correct.

The simple example with which I was testing would be this (It's a class I did):

class ConnectPop3
{             
    private string username = "[email protected]";

    private string password = "miclave";

    private int port = 995;

    private string hostname = "pop.gmail.com";

    private bool useSsl = true;

    public List<Message> getMensajes()
    {
        try
        {

            // El cliente se desconecta al terminar el using
            using (Pop3Client client = new Pop3Client())
            {
                // conectamos al servidor
                client.Connect(hostname, port, useSsl);

                // Autentificación
                client.Authenticate(username, password, OpenPop.Pop3.AuthenticationMethod.UsernameAndPassword);

                // Obtenemos los Uids mensajes
                List<string> uids = client.GetMessageUids();

                // creamos instancia de mensajes
                List<Message> lstMessages = new List<Message>();

                // Recorremos para comparar
                for (int i = 0; i < uids.Count; i++)
                {
                    //obtenemos el uid actual, es él id del mensaje
                    string currentUidOnServer = uids[i];

                    //por medio del uid obtenemos el mensaje con el siguiente metodo
                    Message oMessage = client.GetMessage(i + 1);

                    //agregamos el mensaje a la lista que regresa el metodo
                    lstMessages.Add(oMessage);

                }

                // regresamos la lista
                return lstMessages;
            }
        }

        catch (Exception ex)
        {
            //si ocurre una excepción regresamos null, es importante que cachen las excepciones, yo
            //lo hice general por modo de ejemplo
            return null;
        }
    }
}

And the index I call it like this:

protected void LeerCorreo_Click(object sender, EventArgs e)
    {

       ConnectPop3 oCP3 = new ConnectPop3();

        //invocamos el metodo para obtener mensajes
        List<OpenPop.Mime.Message> lstMensajes = oCP3.getMensajes();

        //recorremos y mostramos el asunto
        foreach (OpenPop.Mime.Message oMensaje in lstMensajes)
        {
            //Console.WriteLine(oMensaje.Headers.Subject);
            MessageBox.Show(oMensaje.Headers.Subject);
        }
   }

I would like to know how to solve the message Server did not accept user credentials. Then I would like to be able to read the emails that have not been read in the inbox, but from those emails, only the attachments with a specific format.

Then I tried this way: I'm currently testing it like this:

       ImapClient ic = new ImapClient("imap.gmail.com", "[email protected]", "miclave",
        AE.Net.Mail.AuthMethods.Login, 993, true);                
        // Select a mailbox. Case-insensitive
        ic.SelectMailbox("INBOX");
        Console.WriteLine(ic.GetMessageCount());             
        System.Windows.Forms.MessageBox.Show(ic.GetMessageCount().ToString());

        MailMessage[] mm = ic.GetMessages(0, 10);
        foreach (MailMessage m in mm)
        {
            Console.WriteLine(m.Subject);
            System.Windows.Forms.MessageBox.Show(m.Subject);
        }
        // Probably wiser to use a using statement
        ic.Dispose();

but I need to read the ones that have not been read from the inbox, but in this case I read them all.