Actually, I've found the email client to be greatly improved in the 2006 OS beta. The thing I most dislike about it at this point is how there doesn't seem to be one an option to keep mail from multiple accounts seperate -- everything just gets dumped into a single inbox.
That's strange, I remember seeing an option for marking mail read/unread (although I guess I could be misremembering) and when I delete messages I get a dialog box asking if I want to delete the mail from the server as well as from the handheld or only from the handheld. Maybe these features are only available for IMAP4 accounts? ...That would be a pretty strange situation, if true.