I noticed that with Thunderbird there seemed to be a 10,000 email limit on some of my accounts (I have a very old AOL account and a fairly old Yahoo).
This seemed annoying but no more until I tried adding those accounts to my updated Outlook client (now 2021).
At that point the 10,000 limit became a real pain.
Yes, both accounts could do with a bit of "tidying" but with everyone in the world trying to email me with "intersting offers" the accounts were well in excess of 10,000.
I could see all the emails in Yahoo's webmail but actually a real email account suits me better.
So....
A few searches later:
Confirmation of the problem
Why is an IMAP Inbox Only Displaying 10,000 Items When There are Many - Microsoft Community
and, maybe a fix
Download your email from Yahoo Mail with IMAP | New Mail for Desktop Help - SLN28681
Thanks to
RESOLVED - Yahoo!/AOL IMAP collection emails per folder limits (fookes.com)
The key change is to update the incoming server name:
export.imap.mail.yahoo.com
(replacing imap.mail.yahoo.com)
for AOL
https://help.aol.com/articles/download-your-email-from-aol-mail-with-imap
similarly
export.imap.aol.com
Hope it helps.
PS
Both needed application specific passwords generated for Outlook 2021.
