TCP/IP vs PPP

I've used TCP/IP since my first days with Windows. But I am now using PPP on the Mac and it seems to be working alright using my old smtp input data. But I've noticed something interesting...when my messages download all seems normal until I boot up my old Windows machine and it's e-mail program. When I start the e-mail program it downloads the SAME messages that just downloaded to the Mac! I can keep all my messages on both machines which is cool. It's almost like having your computers networked. So can anyone explain what processes are taking place here? I'm all ears.

I use Thunderbird, and in that application's server settings is a box I can check off if I prefer to leave the emails on the server. I don't, so it isn't checked.

Apple's Mail takes the opposite approach. Under the Accounts tab, Advanced, the choice is "Remove copy from server after retrieving a message." I want it to, so it's checked.

Do you know the size limit of your server's in-box? If you never clear it, it may reach its capacity and bounce all further mail, unless the PC's email program is set to clear the messages from the server.

I found the tab, thanks. Since I'm still using my Windows machines I don't think I'll have any trouble making sure all the messages get downloaded off the server. I'm still curious as to the nature of PPP and what, if any, are advantages over TCP/IP.

According to this site (I keep it bookmarked), PPP sends the computer's TCP/IP packets to a server that puts them onto the Internet.