You know you've got a good spam filter when…

…users start complaining if a spam gets through.

Human beings are incredibly adaptable — we can learn to live with (almost) any change, while only really noticing and complaining about the moment of change itself. User reaction to spam filter effectiveness is a case in point.

If you've got a moderately good spam filter, which lets through a couple a day, users will quietly and uncomplainingly adapt. They'll internalize "spam? oh, hit the delete key" as part of their unconscious daily routine. But if you upgrade to a really good spam filter, you're in trouble. Users will adapt to the absence of spam — they'll throw away that part of their routine, even though they don't realise it. And then when a spam does get through, hoo boy do they notice it.

This morning a Nigerian-style spam made it through our filters and into a lot of Microsoft inboxes. Suddenly there's discussion on umpteen threads, tips on how to report spam, notification that the spammer account has already been shut down, and generally a lot of attention from people whose routines have suddenly been jolted.

Change. It's what's for dinner.