A good 50% of my 400 incoming messages a day are spam. At first it did not seem like a big deal just to delete them and then I realized how inefficient this actually was. Thus, I made use of Spam Assassin, offered free by my web host. As online booksellers a good portion of your time my be spent in your inbox, so this could be a crucial element to your day-to-day productivity levels. I just found a pretty decent link on how to set-up your Spam Assassin account, step-by-step. Rather than have Outlook automatically detect incoming spam, I try and keep spam off my local machine all together and filter anything marked as spam to a spam email address.

If you do not have hosted email and use free web-based accounts, I suggest Gmail. They have a constantly growing knowledge base of spammers and the reporting spam function is easy to use (also, they recently made it easy to delete all spam messages in one fell swoop).