Fastmail user here. I had problems with spam too when I first signed up, but then I discovered I could change the spam protection level from "Standard" to "Aggressive" in the mail settings screen (the default is "Standard"). That immediately solved almost all of my spam problems.

Unfortunately the problem wasn’t just spam in the inbox it was also non-spam in the spambox

I never had that problem, unless you mean things like the occasional Facebook or Twitter notification ending up in spam, but I didn't care about those.

Also, Gmail definitely has a problem with false positives. At the small agency where I worked a few years ago, a lot of our internal company emails were ending up in spam folders. It got so bad that I had to go around adding our company email domain to the spam exclusion filters just to make sure that my co-workers were receiving my emails.

I'd rather be convinced by the benefits of something than the disadvantages of what I'm already using. For me, Gmail works. Fastmail did not.

I'm not attacking Fastmail. I just didn't want to put in the effort to train the spam filters.

Could you explain what this means?

I assume gp is referencing the necessity of training the spam filter.

I didn't have big problems with it, except for a monthly student loan payment confirmation email that fastmail refused to classify as not spam no matter how many times I so marked it.

In general, I found gmail's spam filter to be the best. If, however, you don't need to regularly interact with lots of new email addresses, fastmail will work fine for you.

This is exactly it. I found myself having to manually classify spam/notspam for the first time in over a decade.

This might be fine for some or even most people but if like me you’d prefer to spend less time on email not more it may not be the best choice as an email provider.

Nothing against the service, just be prepared to manually train a spam filter