//get all messages
                POP3_MessagesInfo infos = pop3.GetMessagesInfo();
                 foreach (POP3_MessageInfo info in infos)
                {
                    byte[] bytes = pop3.GetMessage(info.MessageNumber);
                    Mime mime = Mime.Parse(bytes);
                    HandleMail(mime);
                    //delete it at last
                    //pop3.DeleteMessage(info.MessageNumber);
                }
                //the second way to do it
//for (int i = 0; i < pop3.Messages.Count; i++)
                //{
                //    byte[] bytes = pop3.Messages[i].MessageToByte();
                //    Mime mime = Mime.Parse(bytes);
                //    HandleMail(mime);
                //    //delete it at last
                //    //pop3.DeleteMessage(pop3.Messages[i].SequenceNumber);
                //}。
。。。
取得的邮件可以这要给获得。
   #region pop3
            //string customer = mime.MainEntity.To.ToAddressListString();//cargo company
            //string sender = mime.MainEntity.From.ToAddressListString();//this is customer who send

            #endregion
            string customer = MailboxesToString(envelope.To);//cargo company
            string sender = MailboxesToString(envelope.From);//this is customer who send

。。。
除此之外，它提供的另外一个工具是IMAP，它操作起来更加方便。代码如下：
。
  IMAP_Client clnt = new IMAP_Client();
            try
            {
                clnt.Connect("mail.xx.com", 143, false);
                clnt.Authenticate("user", "password");
                string[] folders = clnt.GetFolders();//get all types

                string folder = "Inbox";
                clnt.SelectFolder(folder);

                IMAP_SequenceSet sequence_set = new IMAP_SequenceSet();
                // All messages
                sequence_set.Parse(string.Format("{0}:{1}", 1, clnt.MessagesCount));

                IMAP_FetchItem[] fetchItems = clnt.FetchMessages(
                        sequence_set,
                        IMAP_FetchItem_Flags.UID | IMAP_FetchItem_Flags.MessageFlags | IMAP_FetchItem_Flags.Size | IMAP_FetchItem_Flags.Envelope,
                        true, false
                        );
                //int count =0;
                foreach (IMAP_FetchItem fetchItem in fetchItems)
                {
                    IMAP_Envelope envelope = fetchItem.Envelope;
                    //hanldle it, means read and search and reply
                    try
                    {
                        HandleMail(envelope);
                        //count++;
                    }
                    catch (Exception ex)
                    {
                        Log("Sys", ex.Message);
                    }
                }
                //delete it after hanlde
                clnt.DeleteMessages(sequence_set, false);
                //disconnect
                clnt.Disconnect();

                //MessageBox.Show(count.ToString() + " of " + fetchItems .Length+ " Success");
            }
            catch (Exception x)
            {
                Log("Sys", x.Message);
                //MessageBox.Show(x.Message);
            }
 
。。
上边都是如何收邮件。
关于如何发送邮件是比较简单的。这里提供两种方式。
第一种方式是使用网上的smtp。这种方式必须要提供用户名和密码。这个适合于web应用，使用的smtp也是网上的，我一般使用163的smtp，基本没有问题。
第二种方式是使用本地的smtp。不需要提供密码，用户也可以是不存在的（垃圾邮件是不是就是这么产生的？），但是必要要提供smtp端口号。
第二种方法测试的时候有时候会报错“Mailbox unavailable. The server response was: 5.7.1 Unable to relay for xxx”，查过资料之后（在baidu上没找到，还是google信息多一点），才发现问题原来是IIS里SMTP服务的配置有问题。
这样修改可以解决：到开SMTP属性—>Access页面?Reply Restrictions/ Reply-?Only this Below选项，加上自己的ip：127.0.0.1（允许本机，使用loalhost，如果是允许其他机器，类似设置）
代码如下
public class EMail
    {
        static public string accountName;
        static public string password;
        static public string smtpServer;
        static public int smtpPort;

        /// <summary>
        /// need password,username, smtpserver
        /// </summary>
        /// <param name="to"></param>
        /// <param name="subject"></param>
        /// <param name="body"></param>
        static public void SendMail(string sendTo, string subject, string body)
        {
            //.net smtp
            System.Web.Mail.MailMessage mailmsg = new System.Web.Mail.MailMessage();
            mailmsg.To = sendTo;
            //mailmsg.Cc = cc;
            mailmsg.Subject = subject;
            mailmsg.Body = body;

            //sender here
            mailmsg.From = EMail.accountName;
            // certify needed 
            mailmsg.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", "1");//1 is to certify
            //the user id  
            mailmsg.Fields.Add(
                "http://schemas.microsoft.com/cdo/configuration/sendusername",
                EMail.accountName);
             //the password
            mailmsg.Fields.Add(
                "http://schemas.microsoft.com/cdo/configuration/sendpassword",
                 EMail.password);

            System.Web.Mail.SmtpMail.SmtpServer = EMail.smtpServer;
            System.Web.Mail.SmtpMail.Send(mailmsg);

        }

        #region send mail2
        /// <summary>
        /// need username,smtp,smtp port
        /// </summary>
        /// <param name="sendTo"></param>
        /// <param name="subject"></param>
        /// <param name="body"></param>
        static public void SendMail2(string sendTo, string subject, string body)
        {
            System.Net.Mail.MailMessage msg = new System.Net.Mail.MailMessage();
            msg.To.Add(sendTo);
            msg.From = new System.Net.Mail.MailAddress(accountName );

            msg.Subject = subject;
            msg.SubjectEncoding = System.Text.Encoding.UTF8;
            msg.Body = body;//
            msg.BodyEncoding = System.Text.Encoding.UTF8;
            msg.IsBodyHtml = false;
            //msg.Priority = MailPriority.High;//

            System.Net.Mail.SmtpClient client = new System.Net.Mail.SmtpClient();
            client.Host = smtpServer;
            client.Port = smtpPort;
            //client.Credentials = new System.Net.NetworkCredential("user@xxx.com", "pass");
            client.Send(msg);

        }
        #endregion

    }