In the first few weeks of using a spam filter, we know it's an inconvenience to have to approve senders when it would be obvious to anyone but a computer that it's mail that you want, but you'll be grateful to have the spam filter when you see all the clutter gone from your mailbox. Note that what appears in your quarantine is only the borderline spam content and that the rest of it is blocked in a "black hole". On average, across all the clients we actively manage, around 80% of all incoming mail is blocked or quarantined as spam!
Technology Advisor Blog
Here's a question we hear a lot: "Why doesn't my spam filter block ALL my spam?" We typically implement Postini spam filtering for our clients (sometimes Barracuda), and when we examine the monthly reports, overall, about 80% of incoming mail is either blocked or quarantined as spam. This is a HUGE amount of mail, and consistent with industry reports which typically track even higher in the 90% range. The filters, though not perfect, keep a LOT of clutter out of your inbox and off your smart phone.