If you monitor these things, you might have noticed that Apple's consumer email domains (iCloud domains) -- mac.com, me.com and icloud.com -- have moved to a "p=quarantine" DMARC policy. This means that if you have an email address in these domains, your ability to send outbound mail using an email service provider or other, non-Apple email platform to send mail, deliverability won't look so good. Mail may not be blocked outright (Apple didn't move to "p=reject") but moving to "p=quarantine" means it's much more likely that your mail could end up in the spam folder.
What to do if you have a mac.com, me.com or icloud.com email address: Continue to send mail, but only from your proper email client on your Mac or iOS device.
What not to do: Don't try to use an ESP to send mail with a from address in the mac.com, me.com or icloud.com domains. It'll fail necessary authentication checks and Apple's DMARC policy will drive most ISPs to put your mail in the spam folder.
I think this is a good move for Apple and a good move for people who hate phishing and spoofing. Making it harder for bad guys to misuse your domains is a good thing.