My friend Tom Murphy and I use the same spam-blocking software. When you install it, Qurb scans your Outlook folders and generates a whitelist of acceptable recipients. Every other email goes into a Qurb folder, which I have to scan regularly to ensure that I’m not missing emails from new contacts. I like this approach because I get no spam in my Inbox, and don’t have to bother ‘teaching’ my email program what is spam and what isn’t. For now, it’s an adequate solution. However, Tom makes a good point regarding the Qurb folder:
I probably get around 400 spam messages overnight, every night. With that volume of spam, I have to rely on the subject line to decide whether to hit the delete button or not. E-mails that have blank subjects or have subjects such as “Hi”, “Hello”, “Information for you” etc. are deleted.
There have been a number of e-mails that I’ve deleted recently from friends because of the subject line. I’m sure this is the same for everyone. This subject line descision making underlines the importance of thinking clearly about the subject of your e-mail. If your e-mail looks like spam it’ll be deleted. It’s worth taking half a minute to think about that when you’re sending an important e-mail to a new contact.
If you’ve emailed me and I’ve never replied, it’s probably not because I’m rude. I’ve just missed your email in my regular parsing of the Qurb folder.
UPDATE: Another signifier is the gibberish email address. I scan the Subject column first, and then if I’m unsure about an email, I take a look at the email address. Most of the time it’s ‘jyuirj@attbi.com’, which confirms that it’s spam.
Bonus link: While scanning ye olde Qurb folder, I spotted the subject line ‘snowmobile calendar’. Given the recent trend of nonense terms in the subject line of spam mails (beetle expense coagulate harvest! destinate hawkins buckskin! button cynthia!), I almost skipped it. Something made me look, though, and indeed it was a legitimate offer. I give you http://www.xtremesnowbabes.com. A calendar of scantily-clad girls on snowmobiles for a mere US $9.99. Apparently ‘this is the only thing Sledding [sic] has been missing’. How Canadian.
DarrenBarefoot.com 
There’s a program that I use called K9 which works really well. You have to teach it what’s spam and what’s not, but I think it’s worth the effort. I’ve never had it trash legitimate messages and I now don’t get any spam in my inbox.
http://www.keir.net/k9.html