Apple's System Status page currently says the following:
Some users were affected
Users may have been unable to send iCloud mail.
Despite this, I was still having problems sending, probably because I monkeyed with the outgoing server settings yesterday when it refused to send.
However, I figured out how to undo all that:
1. I pulled everything out of my Apple Mail client's Inbox and saved it On My Mac.
2. I went into my Apple Mail client preferences/accounts/ and made sure the iCloud email account was highlighted in the menu list on the left side of that preferences window, then, I selected the "Server Settings" tab and changed the pulldown menu for the outgoing server to "none."
3. Still in the Apple Mail client preferences window for the iCloud account, I selected the "Account Information" tab and removed the checkmark beside "enable this account."
4. I quit out of the Apple Mail client.
4. I opened System Preferences/iCloud intending to remove the checkmark beside "Mail" but found that it was already unchecked (because, in Step 3, I disabled the account in the Mail client preferences).
5. I rechecked the box beside "Mail" in System Preferences/iCloud, waited a moment, then closed out of System Preferences.
6. I reopened the Apple Mail client and was able to send email using my BLAH[at]mac[dot]com email account.
It's likely that some of those steps were unnecessary, but as I am not savvy enough to know which, I've listed everything I did here in the hope it might help others with the same problem.