I experienced the same thing. I switched a couple of domains over to iCloud and within a week moved them elsewhere. I went from pracically no spam to hundreds of blatently obvious and easy to catch spam emails hitting the inboxes every day.
I dont know what Apple uses for spam protection, but its total junk. I could roll a better spam protection myself!
As far as I can tell, they are still using Oracle’s flavor of the Sun Internet Mail Server, which was such a piece of unfettered crap when I had to run it in 1999-2004 that we had to set up proxy MTAs in front.
What client do you use? I switched to iCloud and started using the Apple Mail app for its integration with Hidden Email Addresses, but the app is totally half baked with tons to weird behaviours (ex. Send an email. A week later no response, click reply on the email and now you are emailing yourself. Ok, so click Reply-all and now you need to delete yourself and move the original recipient to the To line. No one-click way to start a follow up email!)
I’ve used Apple Mail on iOS and Mac exclusively for about 5-6 years. Maybe my workflow is just adapted but the only issues I’ve had were the macOS version occasionally locking up or giving inconsistent search results usually solved by restarting the app.
I dont know what Apple uses for spam protection, but its total junk. I could roll a better spam protection myself!