Using classic Outlook for Windows in business environments
Understand the frustratuon (we are all having it); but this does not appear to be a Microsoft Outlook and/or Office 365 issue. There as been prior discussion on that issue in this forum. see also:
https://www.reddit.com/r/Thunderbird/comments/v4ydu0/10000_email_limit_via_imap_in_yahoo_email/
This appears clearly to be a Yahoo issue, but going deeper perhaps those that provide service to Yanoo.