First, and generally, SMTP mail is and will be fundamentally broken, and is and will be fundamentally unreliable, and is and will drop messages. The combination of spam and the limits of the forty-some year old SMTP mail design have rendered mail less than reliable. This mess isn’t going to get appreciably better, either. And it likely will get worse, thanks to the assistance of AI slop. You can thank the spammers.
This previous paragraph is a whole lot of words for “use some other path to send your needs-to-arrive messages”.
As for the gobbledygook, Microsoft is reporting that one or more of the three different and ad-hoc methods used to try to differentiate legitimate mail servers (SPF, DKIM, DMARC) from spam servers are incorrectly configured on an unspecified SMTP mail server within Apple’s immense iCloud infrastructure, or that the domain within the SMTP return-path address (which Microsoft helpfully calls 5321.MailFrom address, P1 sender, or envelope sender) matches the domain in what Microsoft helpfully calls the 5322.From address, or that these or other Microsoft checks for these or other mail server features are somehow malfunctioning, and the Microsoft SMTP mail server is rejecting the arriving mail message.
This previous paragraph is undoubtedly also reading like condensed gobbledygook.
There is exactly nothing any of us here can do anything about the spam mess, and about the Microsoft anti-spam checks, and about the Apple iCloud SMTP server settings, too.
You can ask Apple Support about this, and they can read the SMTP message headers from the rejected message to see which Apple mail server is involved, and to verify its settings, but this can just as easily be a malfunction in the Microsoft anti-spam implementation, or a general failure elsewhere in the connection.
I’m surprised SMTP mail still works as well as it does.
Contact Apple Support. Expect to provide some details, and probably the SMTP mail headers from the Microsoft rejection.
