说明：  收邮件的协议有两种，一种是POP3，另外一种较为高级的协议是IMAP4，有的邮箱使用的是POP3协议，比如网易163免费邮箱、新浪免费邮箱、搜狐免费邮箱，有的邮箱使用的是IMAP4协议，比如Gmail免费邮箱，有的两种都可以，比如QQ邮箱。这些免费邮箱都需要做一些设置（例如选择开启 POP3）才能开启从客户端（比如自己做的接收邮件的程序）接收邮件的功能
(private Pop3ConnectionState Connect(string host, int port, SslHandShake sslHandShake, bool ssl) { OnConnecting() this._host = host this._port = port this._ssl = ssl this._tcpClient = this.GetTcpClient(host, port) if (this._tcpClient == null) { return Pop3ConnectionState.DISCONNECTED } this._stream = _tcpClient.GetStream() if (ssl) { //do ssl handshake if (sslHandShake == null) sslHandShake = new SslHandShake(host) DoSslHandShake(sslHandShake) if (_sslStream == null || _sslStream.IsAuthenticated == false) { this._connectionState = Pop3ConnectionState.DISCONNECTED this._tcpClient = null return this._connectionState } this._st)