We were hosting our own email for a good long while and simply couldn't keep up with the spam without spending a vaster sum of money than we wanted to spend, so we decided to let our ISP do the hosting. So far, we've had very good success with the spam filters provided by our ISP. On my machine alone, Postini (see Guest Dale's post, above) catches and quarantines app. 800 pieces of spam a week -- in that same week, maybe three or four will sneak through to my inbox and, on average, fewer than one false positive per week. For my purposes, not bad at all. Similar results on the other dozen machines in the office. At home, my ISP uses a filter called Spasm, which performs similarly.