I have two domains with several addresses which date back to the mid 90s hosted at Fastmail and their spam filtering has been outstanding in my experience.
I’ve been using Fastmail for nine years. It has been excellent on handling spam until about a month ago and I’m now getting a number of false negatives (spam in my inbox instead of in my spam folder). I’m not sure what’s changed but I’m not happy with their current spam filtering.
I have been happy with Fastmail’s spam filtering. It can also be tuned. I end up with virtually no spam in my inbox. If you listen to MacGeekGab they had a couple discussions during July about spam filtering in general and Fastmail’s capabilities were discussed a bit.
I used Spamassassin for a few years in the early 2000s on our company email server and replaced that with a Barracuda spam firewall. Both did a good job but neither has been able to match Google.
I have occasionally blocked a vendor that won’t honor an unsubscribe request, but I can’t remember the last time I received any real spam in my inbox.
I am having a hard time thinking of what the experiment design would be to test the hypothesis that Fastmail better than Gmail (or visa versa).
I use Fast mail extensively now and for many years and my numerous email addresses are out there and I notice few spam sneaking thru and few false positives. I realise that only an assertion and not a tested and proven hypothesis.