Apple's iCloud Mail is a top ten consumer email mailbox provider based in the US, hosting consumer mailboxes at the domains mac.com, me.com and icloud.com.
Apple may not always make it clear exactly why they may have blocked your mail, but I strongly believe that they look at the typical deliverability and reputation-related data points that most smart ISPs look at. Based on metrics and reputation, do they suspect that the mail you are sending is unwanted? Do they see high spam complaints? Are you blacklisted by Spamhaus or is your mail fingerprinted as spam with a major reputation provider such as Proofpoint? Any of these are likely reasons for being blocked from sending to Apple's consumer mailboxes.
Apple does publish a Postmaster page here, where they make it clear that opt-in permission practices are strongly recommended. I'd go so far as to call it a requirement. No list purchases, list rentals, or email appends allowed.
Apple indicates that iCloud mail respects a domain's published DMARC policy. What that means to you is that if your mail fails DMARC checks, that's a reason for Apple to disallow that mail through to their subscribers.
Be sure to read the SMTP bounce responses that you get back from any mail delivery attempts rejected by Apple's mail servers. They may offer insight into what's happening.
Specifically if the bounce is a 4xx temporary error that says something akin to "try again later," I have found that to be correct. They're asking you to try again later because their mail server isn't able to accept that email message right now, and this is usually not because of a reputation issue.
If you need to reach out to Apple to request unblocking, information on how to do that can be found on their Postmaster page.
Need help sending mail to Apple's privaterelay.appleid.com domain? Check out: Help! All mail to privaterelay.appleid.com users is bouncing!
Click here to view the whole series of Spam Resource ISP Deliverability Guides.