Spam, spam, spam, spam,
Some spam doesn't appear to have a reason to have been sent. It's not trying to sell you anything in fact many are gibberish and some have no message at all, but at the very least they confirm that the mailbox is live and this is often all the spammer needs to know.
The nasty hackers out there are always looking for unprotected computers to utilise for Denial Of Service attacks. A DOS attack is where hackers take control of thousands of computers and use the sheer numbers to send emails or website requests to a company or organisation and the sheer volume crashes recipients website or mail servers.
There are two market leaders for mail filtering, Mailwasher and Cloudmark. Mailwasher is best because it looks at the email whilst it's on your service providers mail server and deletes anything nasty before it reaches your computer; on the downside it's a nightmare to setup and run efficiently and it's easy for genuine emails to be deleted.
Cloudmark is available in two versions for "Outlook" and for "Outlook Express". Once installed it creates a new folder appropriately called Spam and filters dodgy emails into that folder where you can peek if you think you've lost an email. Cloudmark has filtered 3,700 emails onto my Spam folder in the 3 month period June to September 2007 - not one was a genuine email and about 20 had to be blocked manually.
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