I'm under the impression that gmail's Bayesian spam filter is what I'll term a Demented Ouroborus Algorithm:
• emails are marked as spam and put in the junk folder where the user never sees them
• because the email is in the junk folder, the algorithm takes that as confirmation that it is spam
• The algorithm then increases the spam likelihood score from that sender on that basis, sending more of their emails into their Gmail recipients' junk folders
• lather, rinse, repeat