After setting up a new computer over the weekend, I noticed that my iCloud email account was showing up as the default "From" address in every new email I composed. I don't actually use this email address, so I had to remember to change the "from" address on every email I sent.
I looked in Mail settings, and verified that emails were supposed to be sent from the "Account of selected mailbox". I didn't want to set a default account here, because I do actually want emails to be sent from the currently selected mailbox, unless no mailbox is currently selected.
A little Googling landed me on this gem:
"Instead of trying to drag the account(s) into the desired order in Preferences, drag the account's mailbox to the desired order from the folder menu on the left side of Mail.app.
Whichever account's mailbox is at the top of the list will be the default mailbox."
That did the trick. Simply dragging the account to the top of the inbox list was all it took.