Free email client and anti-spam filter with bayesian spam filtering. yMail can be used as a spam cleaner for your existing email setup and also as a standalone email client.

Spam Filters
yMail's spam filters use Bayesian techniques. You can show yMail which messages are spam and which are not (before you even download them to your computer!) and each time will adjust its filters to suit. After a few days, it will be able to sort mail quickly and efficiently without any input.
Scanning
yMail can also be used as a mail scanner - you don't have to use the email portion of the program, just use yMail to preview and automatically delete spam before downloading into your regular email software.
Run from a memory stick?
Yes. Because you can easily tell yMail where your message store should be, it's possible to run yMail from a removable drive. Nothing is stored on the host computer, it's completely self-contained. Therefore, you can take yMail with you and simply copy your message folders back to your hard drive when you return.
Attachments
For security reasons, yMail doesn't open or display attachments automatically. It warns you if you try and open attachments, and refuses point-blank to run executable files. It won't render HTML emails, so it's proof against spam which runs off and loads web bugs when you preview it (That's one trick they use to confirm your email address is valid)
It also ignores receive notifications, javascript, vbscript and monkeyscript. Ok, I made the last one up, but if they invent a new kind of script or worm, yMail will continue to ignore it. If you DO receive an HTML formatted email you can click a special link to open the message in your default browser, but I recommend against this unless you know the sender AND know what they're sending you. yMail will strip out all the html and show the text in the preview window anyway, so you'll have a pretty good idea of the content AND you won't see flashen-blinken-lights or advertising banners.