Getting Started with POP3 Using Boost Asio - c++

Question

To help myself learn socket programming, I made a very simple SMTP client (see code below), using some code from the the Wt SMTP Library
As an exercise, I thought I would take a shot at implementing a POP3 client. Using the Wikipedia as a reference, it looked as if you must simply connect to the server on port 110 and send commands (just like SMTP connects on port 25 and sends commands).
However, I have so far been unable to even connect, and basically just timeout in the get_response() every time. I tried connecting to pop.gmail.com with no luck.
I don't believe that you need to send any user/password/other information when the connection is first established? I expected to receive something along the lines of +OK POP3 Server Ready once I connect, and I can then send the USER and PASS commands.
My question is: Am I missing something during the connection phase? Am I trying to connect to a bad host (gmail)? If so, is there a better host than pop.gmail.com with which I can test? Or am I going about this entirely wrong?
#include <string>
#include <sstream>
#include <iostream>
#include <iterator>
#include <boost/asio.hpp>
using boost::asio::ip::tcp;
int main()
{
boost::asio::io_service io_service;
tcp::socket socket(io_service);
tcp::resolver resolver(io_service);
tcp::resolver::query query("localhost", "25"); // Change this for POP3
tcp::resolver::iterator endpt_iter = resolver.resolve(query);
boost::system::error_code erc = boost::asio::error::host_not_found;
while(erc && endpt_iter != end)
{
socket.close();
socket.connect(*endpt_iter++, erc);
}
if(erc) { socket.close(); return }
std::cout << get_response(socket) << std::endl; // 220 <my-host-name> ESMTP Postfix
send(socket, "EHLO localhost\r\n");
std::cout << get_response(socket) << std::endl; // 250-<my-host-name>
return 0;
}
std::string get_response(tcp::socket& socket)
{
boost::asio::streambuf response;
for(;;)
{
std::string msg;
boost::asio::read_until(socket, response, "\r\n");
std::istream in(&response);
std::getline(in, msg);
return msg;
}
}
void send(tcp::socket& socket, std::string msg)
{
boost::asio::write(socket, boost::asio::buffer(msg));
}

Answer 1

Gmail requires SSL/TLS encryption on its POP3 and SMTP connections.
For POP3, you have to connect to port 995 and initiate an SSL/TLS handshake immediately upon connecting. Then you can read the server's initial +OK greeting from, and send commands to, the encrypted connection.
For SMTP, you have a couple of choices:
connect to port 465. Initiate an SSL/TLS handshake immediately upon connecting. Read the +OK greeting from, and send all commands to, the encrypted connection.
connect to port 587. Read the +OK greeting from, and send EHLO and STARTTLS commands to, the unencrypted connection. Initiate the SSL/TLS handshake. Send a new EHLO command, and other commands, to the encrypted connection.