If you start thinking about them too much, spam filters become really problematic concepts.
Just a small example, I sent a Gmail invite to my Yahoo account.
I was waiting for it to show up, and it was taking a really long time.
Then it occurred to me to check the Spam box in my Yahoo account. And of course, buried amongst the various sex-infused offers for pharmaceuticals, OEM software, and great wealth, was my Gmail invite.
Nice one, Yahoo. I think it's pretty clear that spam filters are being used as weapons in many contexts. They are not only being used in competition between corporations as in this example, but they are also being used, intentionally or not, as ways to complicate political speech, as it gets harder and harder to get bulk mailings through to people.