I'm pretty sure FastMail uses Spamassassin, so why is an email company so bad at configuring that software? Their spam filtering is simply not nearly as good as

I'm pretty sure FastMail uses Spamassassin, so why is an email company so bad at configuring that software? Their spam filtering is simply not nearly as good as Gmail, it's not even close. I used Spamassassin back in the day and it was effective for a bit, and then pretty much all spammers figured out how to avoid it. Also, I'm not talking about the obvious VIAGRA large caps spam, which I think is fairly easy to solve. But spam has become much more nuanced and Gmail is the best at it, and the false positive rate is still amazing.