It depends what you want really. For the mail client that has everything (apart from decent IMAP support) i'd recommend The Bat! from
www.ritlabs.com. It's amazingly configurable but not easy for the computer uninitiated. For something the plugs in and works i'd go for Netscape communicator (later than 4.7 or it's full of bugs!!) - easy to setup and a damn site more stable than M$ Outlook.
Just my £0.02.....