Comment by patja

I've been running my own MX for my business for a couple decades. I've never had trouble with Google. Apple on the other hand...as expected are not accommodating to anyone operating outside of the Apple garden. They don't even do DMARC reports, and just point you at their policy page with no indication of why they reject some mail to icloud addresses but not others. I will say I have received specific feedback from a human at Apple after submitting an email and waiting about a month for a response. But it was shocking in how braindead and years-out-of-date their reason was for blocking some (not all!) emails from our domain.

Actually the absolute worst are the rinky dink "free email" hosting that is bundled with some cheap web hosting services, where they use the UCEPROTECT block lists. UCEPROTECT is basically a protection racket where they expect you to pay to be removed from their blocklists, and they are often the one and only blocklist a domain or IP will appear on (which indicates it is likely a false positive money grab)