iCloud.com email servers rejecting incoming email 450 4.1.8 "Domain not found" for an active domain.

The iCloud.com email domain is rejecting incoming email with SMTP error 450 4.1.8, which is "Domain not found."

The sending domain has been up and running for over 20 hours, and Google, Yahoo, Comcast, Verizon and ATT email clients can all send and receive email to/from the "not found" domain.

This is also happening for any email address @me.com, @mac.com etc...

What's going on?

Are Apple DNS servers just super lazy? Super paranoid?

This is a big drag because the "not found" domain is for a law firm and they need to be able to communicate with their clients who use Apple email domains!