Similar problem here (since yesterday or earlier)
Using iPhone I can send as the following
myname@me.com
myname@mac.com
If I send as
myname@icloud.com
I receive an error;
'A copy has been placed in your Outbox. The sender address "myname@icloud.com" was rejected by the server.'
Test emails, sending as those first two addresses, are received without issue on all my devices
(other devices seem to work the same, but I haven't tested them all)
Using a non icloud email address to send I can send to myname@icloud.com (makes the whoosh sound), but this email never appears on any device, never appears in mail at icloud.com.in safari and doesn't bounce back
This means that anyone trying to contact me using myname@icloud.com will never succeed in getting a message to me and never receive any indication of this failure
Nothing has changed on any of my devices and I'm certain that logging out and in is not going to help since the issue exists at icloud.com
My advice is not to attempt to change any settings if you are sure it was working previously AND you haven't changed anything.
I often see email errors (at the server) cause people to change settings in a vain attempt to fix things. When the server issue disappears those who left settings alone will no longer have problems. Those who altered things might experience ongoing issues until their own settings are manually restored. (this is why I love iCloud on iOS - only a user name and password with all other settings handled by Apple/iOS)
Since Apple wont react to this post it is best to wait a day, test again and then call Applecare
