Official Gmail Blog: The mail you want, not the spam you don’t
For starters, the spam filter now uses an artificial neural network to detect and block the especially sneaky spam—the kind that could actually pass for wanted mail.
We also recognize that not all inboxes are alike. So while your neighbor may love weekly email newsletters, you may loathe them. With advances in machine learning, the spam filter can now reflect these individual preferences.
Finally, the spam filter is better than ever at rooting out email impersonation—that nasty source of most phishing scams. Thanks to new machine learning signals, Gmail can now figure out whether a message actually came from its sender, and keep bogus email at bay.