Question

Why is continuous configuration of anti-spam filters necessary within an organization?

Answer 1

Because the companies sending out the spam are also continuously adjusting their messages to get around the spam-filter rules that are in place

Answer 2

I thought bayesian spam filters update themselves :(

Answer 3

Even bayesian spam filters have their disadvantages ( http://en.wikipedia.org/wiki/Bayesian_spam_filtering#Disadvantages )
and so I'm sure more effective techniques will be discovered, and then more effective ways around them. The cycle continues...