As a newsletter provider, I can say that iCloud is problematic. People who actively want our newsletters often have trouble getting them. iCloud will bounce their emails back for no good reason.
Eventually our subscribers will get in touch with us asking why our emails are no longer getting sent to them. This is with very low spam rates, DKIM, and DMARC set up. There's also no good way to contact the iCloud postmaster, which is an option for every other major email service.
In case it's helpful, https://support.apple.com/en-us/102322 has a handy bulk mail deliverability checklist and the direct email of the iCloud Mail postmaster team.
System admin can get direct support from Apple's iCloud Mail postmaster team at icloudadmin@apple.com. https://support.apple.com/en-us/102322 has a list of the information they want in order to diagnose and address delivery issues.
Eventually our subscribers will get in touch with us asking why our emails are no longer getting sent to them. This is with very low spam rates, DKIM, and DMARC set up. There's also no good way to contact the iCloud postmaster, which is an option for every other major email service.