I'm a big Gnus fan. It's been my newsreader for about 10 years, and my mailreader almost since it started supporting mail.

I wouldn't recommend it to an Emacs newbie, but you're obviously not one, so that's OK.

It runs anywhere you have Emacs, so that's your cross-platform portability layer taken care of.

I'm currently running 5.8.8 that comes with Debian woody; its IMAP and MIME support is reasonably complete now (and it has useful features like the ability to read text/plain in preference to HTML in multipart/alternative). I've been using the IMAP backend for a couple of years now, on Linux and previously on Windows.

I've no experience of the Gnus Agent (offline support), and I've never had need for anything beyond simple searching in one group - sorry, folder.

Basically, it meets all your requirements and at least your first wish, possibly more.

Like the empeg, Gnus's greatest asset is its active developer and user communities, with a mailing list and newsgroup respectively.

HTH,