can't send to @mac.com

I can send email to anyone from iCloud. If I try sending email to anyone at an @mac.com address, it is rejected

Your message cannot be delivered to the following recipients:

Recipient address: XXXXXX @mac.com
Reason: unknown or illegal alias

Mind you, I go to any other mail application and it sends just fine.

It rejects it immediately as well. This is when trying to share an iWork app or just a regular email from iCloud.

The response is instaneous, so it's something definitely on the iCloud side of the Mac servers.

Anyone else running into this?