I had that problem, and fixed it by doing these four things:
* Train the built in Bayesian filter fully
* Setup SPF
* Setup DKIM
* Point MX records at Fastmail directly (rather than forwarding via some other service)
More of a pain than GMail which often "just works", but still relatively simple to setup. I suspect ultimately more robust than using mail forwarding on consumer GMail (the rules of which constantly change and it gets less powerful, if you don't buy GSuite and point your whole domain at it).
* Train the built in Bayesian filter fully * Setup SPF * Setup DKIM * Point MX records at Fastmail directly (rather than forwarding via some other service)
More of a pain than GMail which often "just works", but still relatively simple to setup. I suspect ultimately more robust than using mail forwarding on consumer GMail (the rules of which constantly change and it gets less powerful, if you don't buy GSuite and point your whole domain at it).