Hi JoeBlack_,
Thank you for your detailed information.
I understand how inconvenient it is to manually delete emails, especially with such a large volume. Based on our tests, it appears that the default setting in the new Outlook only selects visible emails (up to 75) when you use the "Select All" option. This limitation makes it challenging to delete a large number of emails at once.
Given that your storage is 96% full, this might be the reason why the rule to delete emails is not running successfully. I recommend emptying items in deleted folder (Right click on the deleted items folder > Empty) and clearing emails in the Recoverable Items folder (Go to Deleted Items and Recover items from this folder, and then empty folder) in the new Outlook. If the folder is full, it can prevent the deletion of emails.
For more information about recoverable folder, please check Recoverable Items folder in Exchange Online | Microsoft Learn
In the meantime, could you please let us know which subscription you are using? We will check if the Online Archive feature is available for your account, which could provide a more efficient solution for managing your email storage.
Thank you for your patience and cooperation.
Sincerely,
Luckie Shi | Community Moderator.