Why some people says that aol.com are a proxy and not a e-mail client or mail server?
Thanks for your question caralhete.
AOL is a very different ISP then most of the others out there. They started their service with a proprietary browser back in the early to mid 90's. They have done all that they can to keep it proprietary for reasons that only they know.
Even when you log in to the AOL website through a standard browser, you are sent to a proxy service to get your mail.
It may have some to do with the fact that we are running out of IP addresses in the IPv4 protocol but I thinks it's more to maintain a captive audience. All of the other ISP's seem to manage it without a proxy.
FYI: The IPv4 is now being replaced by IPv6 which is a little more complicated but will add a multitude of new addresses to the mix.