Is there any way we can connect to a POP server through an HTTP proxy? The server is standard POP/SMTP MTA (Mail Transfer Agent), while the client connect from behind a firewalled network with an HTTP proxy setup for internet traffic. Can POP traffic be tunneled through HTTP?
Yes, but you have to have a client and server or some gateway mechanisms that will encapsulate / decapsulate the http wrapper (tunnel). There are client proxies out there (eg- mailFish) that will do this for you.
Much of the time this is used to circumvent corporate firewall policies. Bad juju.
You can place HTTP-frontend to your POP-server, like Squirrel Mail outside your network and connect to it through HTTP. Or just set your Gmail account to fetch mail from POP server and use it.
I am going to assume that you want a "normal" solution.
There is no protocol specification that allows you to configure for a normal "HTTP" (not HTTPS/Connect) proxy, and get your mail.
The proposed alternatives have serious limitations.
If you want to use a native tunneling solution (httptunnel or VPN), you need to have a destination server that decapsulates. In other words, you have to set up a system of servers to circumvent the network security.
If you use something like mailFish, you need to use their server. That means you are giving a third party full access to your email (including password). You are also assuming that your web administrators will not simply block access to that server over HTTP.
You would be better off looking for an ISP that offers a mail service w/ over SSL. Most corporate networks do not block outbound SSL to the typical ports, so you would be both secure-over-the-wire -and- using a valid network connection.