The old account was POP, not IMAP. Therefore, there is no chance that the emails are still on Spectrum servers. I talked with three Apple techs and here's the long and short of it:
The first tech suggested that I delete the problem account and set up a new one. That solved the receiving issue but, unfortunately, she lost ALL her emails. I should have kept that account instead of deleting it. The new account downloads from a POP server but Mail lists it as IMAP. I want to delete the emails from the server as I download them, from Mail, not from Spectrum Webmail. How do I do that?
The second tech (the phone connection disconnected and I had to call again) passed me on to a higher level tech in order to figure out how to get her emails back. Without going into details, Time Machine was unsuccessful. I had a clone that was last updated on July 22 so I figured that I could, at least, retrieve emails from that date. Under the direction of the tech, I used Mail > File > Import Mailboxes to import ~/Library/Mail/V7. This resulted in imported mailboxes. I was able to move the emails from them into my wife's new mailbox.
Now, I have a new issue. I prefer Column view in Mail and I always have the emails spaced above the preview pane. In order to move as many emails as possible at one time, I lowered the space bar so that the preview pane was minimized (actually, not visible). Now, I cannot raise it again. Luckily, I can still double-click an email to open it and view it, but this is less convenient. If I uncheck Column view, I cannot get the side preview pane either, even by checking that option under View.
Any thoughts?
EDIT: At least there is some good news. I checked Spectrum Webmail and, to my surprise, all the old emails are there. This, despite it being a POP account. Go figure.