Not at all. It doesn't have to be 100% of the data, it just has to be enough of the data that it influences the filter's decisions toward one direction over ano

Not at all. It doesn't have to be 100% of the data, it just has to be enough of the data that it influences the filter's decisions toward one direction over another. And since GMail has been in existence for 18 years and has a massive user base, I would be extraordinarily shocked if Google's initial seed training set isn't totally overshadowed by user-supplied data by now.