Q: In Apple Mail, how do I change the default outgoing address for mailboxes that I've set up?

In Apple Mail (Yosemite on a MacBook Pro), the default address for outgoing messages from mailboxes that I've set up that are *not* tied to separate email accounts has suddenly become my iCloud address (which I almost never use). I've got my default set to use whatever address is the default for the mailbox I'm in. This works fine for mailboxes associated with different mail accounts, and until recently it worked fine for mailboxes that I had set up for convenience, using my default email address (which is an Earthlink address I've had for many years). Somewhere within the last year, perhaps with an upgrade to the OS, I've found that outgoing mail from one of those mailboxes (but not from the general Inbox) defaults to my iCloud address. I have to manually choose a different address each time (which is hard to remember to do). This is new behavior, and very unwelcome. But I can see no way to change the default email address associated with each mailbox.

How do I undo this unwanted connection?