Gnus

From Grokdoc

Gnus is a very powerful mail and newsreader based on GNU Emacs. Its goal is to read messages no matter what form they come in, and display them in a generic manner.

There are many "backends" (which retrieve the messages) like RSS, pop, maildir, mbox and even one to fetch Slashdot discussions.

As your requirements grow you can extend Gnus yourself with elisp (Emacs Lisp). This is what makes Gnus stand out from the crowd as you can easily write extensions to do things that no other readers are not capable of.

Though perhaps not ideal for a user migrating from Outlook, Gnus should always be a consideration for power users.