Old Yahoo emails appear as read in Apple Mail

My wife has a Yahoo email account with over 100,000 unread emails in it. Mostly they’re advertisements, promo codes, etc. that date back quite a few years. She has decided she wants to finally clean out the Yahoo mail account and feels pretty confident she can just delete unread emails past a certain point. She doesn’t want to abandon the email account, because it contains some things she wants to save. The things she wants to save have been opened previously and are intermingled with the unread (and unwanted) emails.

The plan was to add her Yahoo account to Apple Mail on her Mac (MacBook Air running Sonoma 14.6.1) and delete unread emails. It appears someone (Apple, Yahoo, whoever makes the IMAP settings, or someone else) limits the inbox to 10,000 messages. (Apparently it’s a thing.) So that’s fine, we can do the delete from Apple Mail in batches.

Here’s the problem: all the old emails are coming in as read. In the Yahoo Mail app and on the web, they show as unread. For clarification, let’s say the moment at which the Yahoo Mail account was added to Apple Mail is Year Zero. Everything that comes in after that (i.e., in AD time), those messages come in correctly as unread. So AD time works as it should. All the messages coming in BEFORE Year Zero (i.e., BCE time), those all (or nearly all) come in as read DESPITE how they show on the Yahoo Mail server.

I installed Yahoo Mail automatically into Apple Mail. (i.e., I selected Yahoo rather than Other when it asked what type of account it was.) I don’t know anything about IMAP settings, so I haven’t started manually tinkering with those (although that is probably what I am going to have to do.)

I have also removed and re-added the Yahoo account a few times.

What can I do to have the old emails (i.e., the ones from the BCE time) come in as unread? (So I can filter for unread and batch delete quickly.)

Thanks.