This post is pretty horrifying. If you're in the Apple ecosystem, Apple Mail and iCloud Mail (standards-based, supports custom domains) has been as reliable as any mail provider and client I've ever used.
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.
I had been happy with Apple for many years but encountered some really terrible experiences lately. There was a systematic problem on Apple’s side (related to the recent increase in iCloud subscription fees) and they cancelled my extra iCloud storage. To be fair they did give me a refund for the extra storage prior but I thought it was related to the pricing changes and didn’t think much of it. Other than that there were no warnings until the extra storage expired and they sent me an email saying the storage is full. I resubscribed immediately but iCloud mail could not send or receive anything for several hours afterwards, except for receiving emails from Apple.
I also have had several instances where iCloud Drive would take forever to sync. The most recent time got so bad (100% cpu usage that persists after killing the process when I added 5KB worth of files) that I stopped using it completely. Tried Microsoft OneDrive instead and it synced at good speeds and gave me no problems.
