My apologies. I did read as far back as your first post, but I missed that the error message was in the subject.
Sorry for jumping in when I had no real help to offer. All I can say is that it is not a universal problem. My iCloud account can send email from icloud.com just fine (not that I use that method except to test it, preferring the Mac or iOS interface).
Before I butt out entirely, allow me to ask one more clarifying question:
Have you tried different recipients and contents in your attempts to send mail?
I ask because one possible reading of the message in the title is that there is something about this particular message that is causing the error.
(For the technically inclined, I'm thinking that some BSMTP return code returned from the recipient site could be described to the user as "this message cannot be sent at this time" if it was a return code Apple didn't feel comfortable retrying. Testing alternative recipients and contents could eliminate this possibility.)