I remember when I was in college and we were doing work about passive OS fingerprinting and we used p0f, Vista was a new OS back then and we fingerprinted it successfully before p0f got its own signatures, it was so cool. It was around 15 years ago, my god time flies so fast.
It’s still great for that exact purpose. Knowing that your SMTP peer is running Windows XP is the strongest spam fighting signal that has ever existed.
