Cross Platform Email Client?
I very highly recommend the email client in Netscape Communicator for the purpose you've described. In the sense of being cross-platform it works very similarly between linux/XWindows and Microsoft's two main windowing products.
I started using it way-back-when because out of the box it could handle HTML. There is a ton about it that I don't like, so every six months or so I get frustrated about something and go look at the others, but I always conclude that this one sucks less than eudora, less than outlook, less than outlook express, less than postilion, less than emacs... I think that's what I've looked at.
I've done the interesting cross-platform things like copy the mailfolders, address books, bookmarks, etc. back and forth and it has worked well whenever I've done it. YMMV because "they" do have a tendency to keep changing the way it works and one never knows when they'll break it.
I've used it in POP3, POP3 "leave on server" and IMAP modes. I would recomment IMAP if your server has it available. It can be confusing to configure (no, I don't want to treat all of my .dotfiles as mailfolders, why are you asking?), but once you get it to work it works the same every day after that.
Perhaps if I get time I'll post more about how I configure it and how I wish I could configure it. One final note: the very latest version silently turned off Emacs-keys but the release notes did mention it and I was able to get them back by ditching the new /usr/X11/blahblahblah/Netscape appdefaults.