According to this article, Outlook Express will no longer be developed. Forcing users to look elsewhere to more updated email clients. Microsoft hopes that users will use either MSN, Hotmail or buy the full version of Outlook.
I say look elsewhere for your mail client, how about Mail.app its robust, has spam filters built in -- but that could be expensive as you need to change platforms (Macintosh). So how about Thunderbird, which is a open source project by the guys that do Mozilla web browser. It also is robust, easy to use and has good learning spam filters. Best of all its free and works on all platforms!
Note: Thunderbird is basicaly the mail client found in the full-blown version of Mozilla, if you use Mozilla (not Firebird) then you do not need Thunderbird as you are basically using it already.
I have to make a quick comment on this, Its odd that Microsoft would stop developing the most used application they have... E-Mail is the killer application for the internet, everyone uses it! Webbased email is great if your on the road without a computer, but Id hate to make it the only way I can read my email. And forcing people to upgrade to Outlook or buy an MSN subscription is just nuts and I'm glad that these alternatives exist.
Update: "Microsoft has backed down on plans to abandon development on its email client, because 'the situation has changed' "