A Guide to Email Clients
Outlook Express
You should ditch Outlook Express. No questions about it. It can execute malicious attachments without your permission, it's clunky, bloated and simply damn unsafe. If you're one of the poor sods to have installed Service Pack 2, then it simply becomes partially crippled. And is still clunky, bloated and unsafe. Fear not, dear reader, for there are plenty of alternatives available.
Thunderbird
Thunderbird is the excellent open source companion to Firefox and is definitely one of the best email programs out there. Unlike M2, it offers great IMAP support and HTML email composing along with a more 'familiar' interface and is possibly the better programme in terms of pure functionality.
M2
If you don't mind a genuinely innovative interface (that some users find confusing), don't ever write HTML email and don't wish to use IMAP on a professional level then I would direct you to M2, the email client integrated into Opera. See here for why it's unique. It's my choice for the reasons given, plus the integration with the browser avoids running multiple programmes and ensures all the Opera bells and whistles (like mouse gestures, user stylesheets etc) work as well.