Af is an Emacs-like electronic mail user agent which is distributed under the terms of the GNU general public license. It is not an Emacs Lisp package which runs under Emacs, it is a stand alone package which happens to have an Emacs-like interface by default.
Af has decent facilities for handling folders: you can open multiple folders at the same time; split the screen in two and display two different folders in the two "windows"; kill messages from one folder and then yank them back into another (cut-and-paste if you're not used to Emacs terminology); and many other things that Emacs users are used to doing with text.
Af also has facilities for handling groups of messages in one go; which can be especially handy if you deal with large-volume mailing lists on a daily basis. Generally, if you need to process large volumes of mail daily, then you may well benefit from using af.
Af is also very configurable (it can do a halfway decent impersonation of several other mailers). You can change what keys do, set variables to change the way things work, and even create your own macros. One day you'll probably be able to write your own commands too, but that's still a way off.
Af is as RFC compliant as possible (barring bugs). It supports POP3, and reading and sending MIME mail. You can use almost anything to send mail: sendmail, smail, qmail, MMDF, or even direct connections to an SMTP server.
All this in a package that seems to be smaller than most full-screen mail readers around. I don't know why it's smaller, since it doesn't lack features, but it is.
If you want to find out more about af, then you can browse the manual for the latest development version.
You can also see the list of people who have contributed to af's development.
Af is not another mail package written in elisp to run under GNU Emacs (sorry to belabour the point, but mention the words "mail" and "Emacs" in one sentence and everyone seems to jump to that conclusion). It simply has an interface that is very similar to Emacs, to make learning to use it simple for Emacs users.
Af is not a package that holds your hand, and leads you through the process of handling your mail. Like Emacs, there are a lot of commands (and mysterious key combinations) in af, and learning to use it well is likely to take some time. That said, Emacs users should be able to do the basic stuff pretty quickly, and relative newcomers have managed to use af quite effectively (this should improve now I've written the manual :-).
Af has several shortcomings (which are basically due to the lack of time on my part). There is no X interface as yet; the extension language is almost entirely incomplete, and various other features I'd like to see one day aren't done yet. I'm releasing it now, partly to see if anyone actually has a use for it, partly to get some feedback about what direction to take in the future, but mostly because the (few) people who use af find it a very good tool as it stands, and I think that it's quite usable now.
Basically, af should run on any modern (ie post-1982) Unix. Until quite recently it was developed on a machine with a 68000 processor, 1.5 MB of RAM and a 40 MB hard drive; so it tends to run quite well on a small machine (with reasonably small folders anyway).
It is distinctly possible that you might have small difficulties in porting af to a new machine, but there shouldn't be any really major problems. You can view the list of systems that af has been tested on.
The rise of the IMAP/SMTP model of mail clients has pretty much made af obsolete - I mean in 2009 even I stopped using it :D Sadly, I haven't been able to dedicate any time to af for over 10 years, so I really have to admit that it's dead.
This page is left here as a kind of technical archaeology; and in the very unlikely case that someone finds it useful.