In the days I ran my own main server for my domain I was surprised to find that well over 50% of spam originated from Google (both emails with @gmail.com domain

In the days I ran my own main server for my domain I was surprised to find that well over 50% of spam originated from Google (both emails with @gmail.com domains but also emails from other domains being processed via their servers). And I wasn't even a Gmail customer, so it looks like they don't really filter outgoing mail to other providers. It made my inbound filtering quite tricky as I couldn't block Google as quite lot of legitimate traffic comes from friends using Gmail so the other Spamassassin rules (e.g. content analysis) had to do much of the heaving lifting. A couple of years back I gave up and outsourced the MX for my domain to Fastmail. Interestingly they also struggle to filter Gmail messages (which isn't too surprising as they also use Spamassassin) but thankfully there are only a few a week that get past and I always make sure these are flagged as spam to train the filter. Over time they end up going into my Spam folder and eventually they just don't arrive at all due to my spam settings blocking high spam scores.