yMail can be used as a spam cleaner for your existing email setup, and also as a standalone email client.

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.

How much does it cost?
Nothing. yMail is completely free to download and use.

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.

Privacy
yMail doesn't render html, doesn't load or display images, and completely ignores return-receipts, so it's immune to web bugs and other tricks for tracking whether you opened an email or not. Your email might not look as pretty as the sender intended, but who cares? If you receive an email you simply have to see in all its glory, you can still view it in your web browser with a click. (I don't recommend it, since this will open you up to all the nasties mentioned before.)

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 yMail 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.

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, java script , vb script  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.

Is it reliable?
I've been using yMail since March 2003 and have yet to lose a single email or experience an unexplained crash, but it might just not work for you. Therefore, no guarantees.